Organic Chemistry - Socorro Independent School District

Organic Chemistry What do we mean by organic chemistry? What do the following things below have in common? Organic-dictionary definition Oxford English dictionary says: organic means derived from living matter, i.e. not produced artificially For example, organic food refers to the food that is produced without the use of chemical fertilisers, pesticides or other artificial chemicals. Organic- chemical definition Of all the elements in the periodic table, one is

much more versatile (stands out) from the rest Carbon can form more compounds than any other element Chemical definition: Organic chemistry is the study of compounds containing carbon (other than simple binary compounds and salts) and chiefly or ultimately of biological origin. Question: Is carbon itself organic or inorganic? Carbon-revisited hopefully! In group 4 of the periodic table Electronic configuration: 2,4 (SL) or 1s2 2s2 2p2 (HL) Has a valency (combining power) of 4 with 4 valence (outer shell) electrons Achieves the noble gas configuration of

neon by forming 4 covalent bonds Take methane for example: carbon is bonded covalently to 4 hydrogen atoms to achieve its octet Carbon- continued The valency of 4 gives rise to a unique property called catenation Catenation: spontaneous linking of atoms of certain chemical elements, such as carbon atoms, to form long chains or stable rings by forming covalent bonds with itself C-C bond enthalpy = 348 kJ mol-1 C-H bond enthalpy = 412 kJ mol-1 Catenation allows billions of organic compounds to be formed Carbon compounds > all other compounds of all the elements in the periodic table except hydrogen (since almost all organic compounds also contain hydrogen)

7 million Organic Compounds 1.5 million Inorganic Compounds Organic chemistry- link to nature Life is based on organic compounds The 4 major classes of biomolecules are all organic (contain carbon) Carbohydrates, Proteins, Lipids & Nucleic Acids Organic chemistry is an excellent foundation for biochemistry Biochemistry is just APPLIED organic chemistry! Just like physics is applied maths! Hydrocarbons A compound which consists of carbon &

hydrogen ONLY Are the basis for most organic compounds Includes alkanes, alkenes, alkynes (aliphatic compounds), & arenes (aromatic compounds) Organic compound familieshomologous series Can be grouped into different families with a common functional group (part of the molecule which gives rise to common reactivity) Homologous series: a group of organic compounds that follow a regular structural pattern and have the same general molecular formula, differing only by the addition of a methylene, -CH2- group, they have almost identical chemical properties, with physical properties e.g. boiling point increasing gradually as the number of carbon atoms increase

Homologous series Take the alkanes for example: The 1st 4 are methane, ethane, propane, butane Homologous series- properties 1) Successive compounds differ from each other by a -CH2- unit (methylene group) 2) The compounds can all be represented by a general formula (e.g. alkanes CnH2n+2; if n=3 then the formula is C3H8 3) The compounds have similar chemical properties 4) Successive compounds have physical properties that vary in a regular manner

as the number of carbon atoms increases Homologous series- general trends in physical properties For the alkanes: As the number of carbon atoms increase the melting/boiling points gradually increase This is caused by an increase in the molar mass of the molecule and hence there is greater chance of more temporary dipoles being induced in the case of hydrocarbons (Van der Waals forces) Graph of boiling points for the first 10 alkanes

Curve is initially quite steep, as for small molecules, the addition of an extra carbon has a proportionally larger effect on the molar mass (eg, from CH4 to C2H6 there is an increase of 97.5%) & hence on the strength of the van der Waals forces As the length of the carbon chain increases, the percentage change in molar masses becomes progressively smaller (there is a 10.9% increase in molar mass from C9H20 to C10H22, hence the curve flattens The trend is the same for other physical properties, such as density & viscosity for the same reasons Empirical, molecular, structural (condensed & full) formulae Using ethane, C2H6, as an example Empirical formula: simplest whole number ratio of atoms e.g. CH3 Molecular formula: actual number of atoms of each type

present in a molecule e.g. C2H6 Full structural formula: show the relative positioning of all the atoms in a molecule & the bonds between them Condensed structural formula: omits bonds which can be assumed & groups atoms together e.g CH3CH3 Shapes of Alkanes Straight-chain alkanes have a zig-zag orientation in 3-D Formulae- worked example Write the empirical, molecular, full structural & condensed structural formula for ethanol, CH3CH2OH

Answer: Empirical: C2H6O Molecular: C2H6O Full structural: Further functional groups Alkenes: CnH2n e.g. ethene C2H4 Alkynes: CnHn e.g. ethyne C2H2 Alcohol: CnH2n+1OH e.g. methanol CH3OH Aldehydes: RCHO, e.g. methanal (formaldehyde) HCHO Ketones: RCOR (R can be the same alkyl group as R or different e.g. propanone (acetone), CH 3COCH3 Carboxylic acids: RCOOH, e.g. methanoic (formic) acid, HCOOH Haloalkanes: RX (X=F,Cl,Br,I) e.g. iodomethane, CH 3I Amines: RNH2 e.g. methylamine, CH3NH2

Esters: RCOOR e.g. methyl methanoate HCO 2CH3 Arenes: based on the phenyl group (C 6H5-) e.g. benzene C6 H 6 Isomers Compounds with the same molecular formula but with different arrangement of atoms in the molecule For the alkanes: from butane (4 carbon atoms), there is more than 1 structure possible C4H10 can refer to either butane, CH3CH2CH2CH3 or methylpropane, CH3CH(CH3)CH3 Structural Isomers

Different connectivity of atoms E.g.: Butane & methylpropane are structural isomers of each other 3 types: 1)Chain isomerism 2)Position isomerism 3)Functional group isomerism Chain isomerism Different arrangement of the carbon skeleton For example butane & methylpropane are chain isomers of each other Have similar chemical properties but different physical ones; the branched isomer

has a lower melting/boiling point than the straight-chained H CH3 H H H H H one H C C C C

H H H butane H H H C C C H H

H methylpropane H Position Isomerism Have functional group placed at a different position along the carbon skeleton E.g. 1-bromopropane & 2-bromopropane Has similar physical & chemical properties H Br H

H C C C H H H 1-bromopropane H

H H Br H C C C H H

H 2-bromopropane H Functional group Isomerism The molecules have different functional groups (hence different chemical properties) E.g. ethanol & methoxymethane all have the same molecular formula of C2H6O, however ethanol is an alcohol while methoxymethane is an ether ethanol methoxymethane

Naming organic compounds International Union of Pure & Applied Chemistry (IUPAC) devised nomenclature method It is irregular for up to 4 carbon atoms, just have the prefixes meth-, eth-, prop-, but From 5 carbon atoms & above the naming becomes systematic, like those of geometrical shapes, e.g. pent(pentagon), hex- (hexagon), hep- (heptagon), oct- (octagon) etc The name of any organic compound is usually made up of 3 parts: prefix (substituents), stem (number of carbon atoms in main chain) & suffix (homologous series of main carbon chain) e.g. methylpropane methyl- CH3 substituent prop- 3 carbon atoms in main chain -ane- belongs to the alkane homologous series Monkeys Eat

Peeled Bananas Counting to Ten in Organic 01 02 03 04 05 06 07 = meth = eth = prop = but = pent

= hex = hept Greek hept) 08 = oct 09 = non Greek non) 10 = dec Mother Enjoys Peanut BUTter PENTagon HEXagon or HEX nut HEPTember (Roman sept is OCTober NONember (Roman nov is DECember

Naming side chains If there are alkyl groups (R groups) attached in isomers, the prefix for the alkyl groups must be used Prefix Name of Name of Alkyl Structure of alkane group alkyl group Meth- Methane

Methyl CH3- Eth- Ethane Ethyl CH3CH2- Prop- Propane Propyl

CH3CH2CH2- But- Butane Butyl CH3CH2CH2C H2- Pent- Pentane Pentyl

CH3CH2CH2C H2CH2- Hex- Hexane Hexyl CH3CH2CH2C H2CH2CH2- Steps in naming alkanes- step 1 For the molecule CH3CH(CH3)CH(CH3)CH3 Identify the longest continuous carbon chain (This may not be the most obvious, straight one), this gives the stem, given by the 4 blue carbon atoms

Steps in naming alkanes- step 2 CH3CH(CH3)CH(CH3)CH3 Identify & name the side-chains/substituent groups as the prefix of the name. Here there are 2 different methyl groups Steps in naming alkanes- step 3 CH3CH(CH3)CH(CH3)CH3 Where there is more than 1 side-chain of the same type, like here, use the prefixes di-, tri-, tetra- and so on, to indicate this. If there are several side-chains within a molecule, put them in alphabetical order, seperated by dashes.

There are 2 methyl groups- hence the prefix is dimethyl Steps in naming alkanes- step 4 CH32CH(CH3)3CH(CH3)4CH3 1 Identify the position of the side chains. The carbon chain is numbered from the end which will give the substituent groups the smallest number. Here 1 methyl group is attached to carbon number 2; the other to carbon number 3. The numbers precede the name and each digit is separated by a comma from the next digit

Hence the name of this compound is 2,3dimethylbutane Some exercises Name the following compound: H H C H H H C H

C H C H H C H H H H

H C C C H H H H 2 methyl

groups Hence green is correct 2 methyl groups Using the green numbering gives the lower substituent number of 3,3dimethylhexane Using the orange numbering gives the higher substituent number of 4,4dimethylhexane 3

4 4 3 2 5 1 6 5 2

6 1 Answer Hence the name of the molecule is 3,3- dimethylhexane H H C H H H

C H C H C H H C H H

H H H C C C H H H

H Methylpropane methylbutane dimethylpropane IUPAC common IUPAC: 2,3,5,4,6-Pentahydroxyhexanal Common: glucose IUPAC: 3-carboxy-3-hydroxypentanedioic acid Common: citric acid

Alkene isomers Have the general formula CnH2n Have the C=C functional group within the chain Simplest alkene is ethene, C2H4 If molecule is longer than 3 carbon chains, the double bond can be in more than 1 position ethene propene Naming alkene isomers Same 1st 4 steps as alkanes except the name (suffix) ends in -ene instead of -ane

Step 5- The position of the double bond C=C is shown by inserting the numb er of the carbon atom at which C=C starts E.g. for the isomers of C4H8 CH3CH2CH=CH2 is but-1-ene CH3CH=CHCH3 is but-2-ene CH2=C(CH3)2 is 2-methylprop-1-ene Practice problem What is the name of the following alkenes: 1)CH3CH=CHCH2CH2CH3 2)CH3CH2CH(CH3)CH=CH2 3)CH2=C(CH3)CH2CH=CH2 Answer: 1)hex-2-ene 2)3-methylpent-1-ene 3)2-methylpent-1,4-diene

Naming alcohols (ROH) Always end in ol Simplest is methanol Like alkenes, the position of the OH group must be specified after ethanol e.g. CH3CH2CH2OH is propan-1-ol CH3CH(OH)CH3 is propan-2-ol ethanol Propan-1-ol Propan-2-ol Practice problems

Name the following alcohols: 1) CH3C(OH)(CH3)CH2CH2CH3 2) Answers 1)2-methylpentan-2-ol 2) propan-1,2,3-triol Naming aldehydes (RCHO) Always end in al NOT (ol)! Simplest is methanal -CHO group is always at the end, so this carbon must be carbon 1 so unnecessary to specify

location E.g. methanal ethanal propanal Practice problems Name the following aldehydes: 1)CH3CH2CH2CHO 2) HCOCH2 CH2CH3 Answer: 1)butanal 2)butanal

Naming ketone (RCOR) Always ends in suffix one Simplest is propanone (acetone) The C=O (carbonyl) group can be inserted anywhere along the hydrocarbon chain except at the end (why?) After butanone, the position of the carbonyl group must be shown E.g pentan-2-one & pentan-3-one propanone butanone Practice problems- ketones Name the following ketones:

1) CH3COCH2CH2CH2CH3 2) CH3CH2CH2COCH3 OC CH3 CH2CH2CH3 Answer: 1)hexan-2-one 2)pentan-2-one 3)pentan-2-one Naming carboxylic acid (RCOOH) End in oic acid Like aldehydes, COOH is always the terminal group & hence this carbon is always carbon number 1

Methanoic acid Ethanoic (acetic) acid Propanoic acid Practice problems- carboxylic acids Name the following carboxylic acids: 1)CH3CH2CH2CO2H 2)HOOCCH2CH2CH3 3)CH3CH2CH(CH3)CH2COOH Answer: 1)butanoic acid 2)butanoic acid 3)3-methylpentanoic acid

Naming Haloalkanes (RX) Have the prefix halo- e.g fluoro-, chloro-, bromo-. Iodo Involve substituting a halogen atom into an alkane Same numbering as alcohols & ketones Numbers must be used after 2 carbon (from propane) atoms E.g. CH3CHClCH3 is 2-chloropropane CH3CH2CHClCH2CH2Br is 1-bromo-3-chloropentane Use the prefixes, di-, tri-, tetra- is there are more than 1 of the same type of halogen atom e.g. chloromethane dichloromethane Trichloromethane

(chloroform) Tetrachloromethane (carbon tetrachloride) Practice problems Name the following haloalkanes: 1) CH3CH2FCHCH2CH3 2) CH3CH2CH2CH2CHBrCH3 Answers: 1)3-fluoropentane 2)2-bromohexane Further functional groups Homologous Function series

al group Amine Esters R-NH2 RCOOR Prefix Amino- Alkyl Suffix

-amine Alkanoat e Example(s) Structural formulae Methylamine CH3NH2 2-aminobutane CH3CH(N H2)CH2C

H3 Ethyl ethanoate CH3CO2C H2CH3 Propyl ethanoate CH3CO2C H2CH2CH 3 Aromatic compounds C6H5-

phenyl- -benzene Benzene C6H6 methylbenzene C6H5CH3 Primary compounds Primary carbon atom attached to a functional group & also to at least 2 H atoms & 1 alkyl (R) group e.g.

H R C H General structure H OH CH3 C H ethanol

OH Secondary compounds Secondary carbon atom attached to a functional group & just 1 H atoms but 2 alkyl (R) group e.g. H R C H OH R General

CH3 C H OH CH3 Propan-2-ol CH3 C OH CH2CH3

Butan-2-ol Tertiary compounds Tertiary carbon atom attached to a functional group & also 3 alkyl (R) groups with no H attached e.g. R R C R General CH2CH3 CH3 OH CH3

C OH CH3 2-methylpropan-2-ol CH3 C OH CH2CH3 3-methylpentan-3-ol CH2CH2CH3

CH3 C OH CH2CH3 3-methylhexan-3-ol Practice questions Are the following molecules primary, secondary or tertiary? 1)1-chlorobutane 2)2-bromobutane 3)2-chloro-2-methylbutane 1-chlorobutane

H H C* CH2CH2CH3 Cl C* attached to 2 H & 1 alkyl group Hence its primary 2-bromobutane H CH3 C* CH2CH3

Br C* attached to 1 H & 2 alkyl groups Hence its secondary 2-bromo-2-methylbutane CH3 CH3 C* CH2CH3 Br C* attached to no H & 3 alkyl groups Hence its tertiary Volatility Measure of how easily a compound evaporates

Depends on intermolecular forces as kinetic energy which hold the molecules together must be overcome Compounds with stronger intermolecular forces will evaporate less readily, hence have higher boiling points The following factors influence volatility: 1)Molecular size 2)Shape of molecule (branched/linear) 3)Functional group Volatility 2 Compounds with longer carbon chains & hence greater molar mass have higher boiling points & lower volatility A molecule with greater molar mass has more

electrons present hence more temporary dipoles can be induced leading to more Van der Waals forces The early members of a series e.g. methane to octane are gases & liquids respectively, at room temperatue The latter members are more likely to be solids e.g. C40- Volatility 3 Also depends on the number of points of contact between molecules Branched isomers usually have lower boiling point/higher volatility than straight-chained linear ones Less surface area for attraction in branched isomers Hence methylpropane has lower boiling

point than butane Volatility 4 Molecules with functional groups that contain hydrogen bonds are likely to have the highest boiling point e.g. ethanol (78C) Molecules with functional groups that contain dipole-dipole attractions are likely to have intermediate boiling points,e.g. ethanal (20.2 C) Molecules which just have van der Waals forces usually have the lowest boiling point e.g propane, (42.1 C) Fair test When comparing molecules for boiling

points/volatility in different homologous series, it is important to compare molecules of similar molar mass This rules out molar mass as being a factor for different boiling points since it becomes the constant variable Hence ethanol (Mr=46) was compared with ethanal (Mr=44) & propane (Mr=44) Solubility in water Depends on nature of functional group & length of hydrocarbon chain Polar functional groups with hydrogen bonding e.g. ethanol are usually the most soluble = most hydrophilic Hydrocarbon chains e.g. alkanes are likely to be least soluble = most hydrophobic Depends on relative length of hydrocarbon chain e.g. very long hydrocarbon chain can overwrite effects of polar functional

group e.g. for the very long chained fatty acids which are solids i.e. butter not oil Alkanes Hydrocarbons General formula= CnH2n+2 Are saturated hydrocarbons Saturated- an organic molecule which only has C-C single bonds & no C=C multiple bonds Unreactive in general Low reactivity of alkanes Only have strong C-C & C-H bonds Need a large energy input to break these strong bonds

Hence stable/unreactive under most conditions C-C & C-H bonds nonpolar as have similar electronegativities (EN= 0.4) Hence no electron-rich or electron-deficient sites 2 main reactions: 1)Combustion 2)Halogenation Combustion of alkanes Very exothermic Due to high strength of C=O in carbon dioxide & O-H bond in water Burns in excess oxygen to give carbon dioxide & water CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890 kJ mol-1 In a limited supply of oxygen, carbon monoxide & water are produced:

CH4(g) + 1.5O2(g) CO(g) + 2H2O(l) In an extremely limited supply of oxygen, carbon itself produced with water CH4(g) + O2(g) C(s) + 2H2O(l) Combustion of alkanes- questions Write the equation for the reaction of propane in excess oxygen, limited oxygen & extremely limited oxygen: Excess oxygen: C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) Limited oxygen: C3H8(g) + 3.5O2(g) 3CO(g) + 4H2O(l) Trace amounts of oxygen: 4H2O(l) C3H8(g) + 2O2(g) 3C(s) +

Halogenation of alkanes If subjected to UV light, methane reacts with chlorine to form chloromethane & hydrogen chloride CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) In excess chlorine, further reaction takes place to produce dichloromethane, trichloromethane (chloroform) & tetrachloromethane Dichloromethane: CH4(g) + 2Cl2(g) CH2Cl2(g) + 2HCl(g) Trichloromethane: CH4(g) + 3Cl2(g) CHCl3(g) + 3HCl(g) Tetrachloromethane: CH4(g) + 4Cl2(g) CCl4(g) + 4HCl(g) Halogenation of alkanes-problems Write the equation for the reaction between

ethane and 1 mol of bromine: C2H6(g) + Br2(g) C2H5Br(l) + HBr(g) General reaction: Alkane + halogen haloalkane + hydrogen halide RH + X2 RX + HX R= alkyl group X= halogen atom Halogenation of alkanes mechanism Substitution reaction (chlorine atoms replace hydrogen atoms in the methane molecule) Substitution reaction- a reaction where 1 atom or group of atoms is replaced by another atom or functional group Mechanism is free radical substitution Degree of substitution hard to control, often mixture of products formed e.g. Ethane reacts with excess bromine to form a

mixture of dibromoethanes: C2H6(g) + 2Br2(g) C2H4Br2(g) + 2HBr(g) Free radicals Species with unpaired electron e.g. Cl ., Br. Extremely reactive Formed by the homolytic fission of a molecule: Cl-Cl 2Cl. (produced by action of UV light) Homolytic fission- the breaking of a covalent bond so that 1 electron from the bond is left on each atom, resulting in the formation of 2 free radicals Free radical substitution mechanism

Is a substitution reaction as a halogen atom replaces a hydrogen/alkyl group. 3 steps: 1)Initiation (net increase in radicals) 2)Propagation (chain reaction- amount of radicals stays the same) 3)Termination (net decrease in radicals) Mechanism in detail UV Initiation: Cl-Cl(g) 2Cl.(g) Propagation: Cl.(g) + CH4(g) .CH3(g) + HCl(g) . (g)

CH3 (g) + Cl2(g) CH3Cl(g) + Cl. Termination: Cl.(g) + Cl.(g) Cl2(g) Cl.(g) + .CH3(g) CH3Cl(g) .CH (g) + .CH (g) C H (g) 3 3 2 6 Overall reaction: CH4(g) + Cl2(g) CH3Cl(g) + HCl(l) Exercise Write the free radical reaction mechanism for the reaction of bromine with ethane: Initiation: Br-Br(g) 2Br.(g)

Propagation: Br.(g) + C2H6(g) .C2H5(g) + HBr(g) . C2H5 (g) + Br2(g) C2H5Br(g) + Br.(g) Termination: Br.(g) + Br.(g) Br2(g) Br.(g) + . C2H5(g) C2H5Br(g) . C H (g) + .C H (g) C4H (g) 2 5 2 5 10 Overall reaction: : HBr(g) C2H6(g) + Br2(g) C2H5Br(l) +

Alkenes Unsaturated hydrocarbons with C=C double bond Unsaturated molecule- a molecule with 1 or more C=C double bonds General formula: CnH2n C=C double bond stronger & shorter than C-C single bond Double bond consists of a stronger sigma () bond & weaker pi () bond Quite reactive due to double bond Take part in addition reactions Addition reaction- a reaction where 2 (or more)molecules combine together to form a single molecule Hydrogenation of alkenes Alkene + hydrogen alkane

E.g. C2H4(g) + H2(g) C2H6(g) H H H H Ni, 180C + H 2 H C C C C H ethene H

H H ethane H Hydrogenation of alkenes-exercise Write the reactions for the hydrogenation of but-1-ene & but-2-ene in the presence of a nickel catalyst & high temperature. 1) With but-1-ene H2C=CHCH2CH3 + H2 CH3CH2CH2CH3 2) With but-2-ene CH3CH=CHCH3 + H2 CH3CH2CH2CH3

NB the original position isomerism is lost in this reaction Used to convert cooking oils to margarine unsaturated saturated Halogenation of alkenes Alkene + Halogen Dihaloalkane Addition reaction H Br Br

H C H ethene + Br2 C H H C C H

H H 1,2-dibromoethane Halogenation of alkenes 2 Alkene + Halogen Dihaloalkane H CH3 C + Cl2 C

H propene H H Cl Cl C C H

H CH3 1,2-dichloropropane Reaction with hydrogen halides Alkene + Hydrogen halide haloalkane Addition reaction H H C H ethene

+ HCl C H H Cl H C C H H

H chloroethane Reaction with hydrogen halides 2 Reactivity order HI>HBr>HCl Weaker strength (longer length) of H-X bond as descending group 7 CH3 CH3 C + HCl

C H but-2-ene H CH3 Br H C C H

H CH3 2-bromobutane Hydration of alkenes Reaction where a water molecule reacts with an unsaturated compound in an addition reaction Alkene + water alcohol RC=CR + H2O ROH In Industry: H H

C H ethene + H2 O C H H H C C

H H H3PO4 300C 60 atm H ethanol OH Also addition reaction- H+ & HSO4- added across double bond H H H

H C H C + H2SO4 H ethene H C H C

OSO3H H Ethyl hydrogensulfate H H C H H C OSO3H H

Ethyl hydrogensulfate H2O H H H C C H

H ethanol OH + H2SO4 Note sulphuric acid is reformedhence its a catalyst Distinguishing between alkanes & alkenes Use halogenation addition reaction to distinguish between these 2 functional groups Used as a test for unsaturation

Addition polymerization of alkenes Monomer Polymer Alkene Polyalkene Monomer Ethene Polymerisation n can be 1000 or more Polymer Poly(e)thene Polymerisation problem Draw the reaction for the polymerisation

of propene: n(Propene) (polypropene)n H CH3 C C H H CH3 C

n H H propene C H polypropene n H H

Cl C C H H Cl C n H C H

Chloroethene (vinyl chloride) n Polychloroethene (PVC) C6H5 H C n H C

H phenylethene (styrene) H C6H5 C C H H n Polyphenylethene (Polystyrene)

Ethene (from cracking) React with steam ethanol polymerise React with chlorine React with benzene Poly(e)thene Chloroethene (vinyl chloride)

Phenylethene (styrene) polymerise polymerise Polychoroethene (PVC) Poly(phenylethene) (polystyrene) Alcohols General formula: CnH2n+1OH Have polar hydroxyl (-OH) group Soluble in water due to hydrogen bonding

from hydroxyl group 2 types of main reactions: 1) Combustion 2) Oxidation Combustion of alcohols Alcohol + oxygen carbon dioxide + water ROH + O2 CO2 + H2O Methanol: 2CH3OH(l) + 3O2(g) 2CO2(g) + 4H2O(l) Ethanol: 2C2H5OH(l) + 7O2(g) 4CO2(g) + 6H2O(l) small Hc medium Hc Propanol: 2C3H7OH(l) + 9O2(g) 6CO2(g) + 8H2O(l) large Hc

Alcohol Ratio Alcohol CO2 Methanol 1 1 Ethanol 1

2 Propanol 1 3 As the alcohol becomes larger the CO2:alcohol ratio is larger, hence more energy is released (due to the strength of C=O bond) Combustion of alcoholsexercises Which of the following alcohols is likely to release more energy upon complete combustion, hexanol or butanol Answer:

Hexanol as has higher molar mass. Alcohols vs alkanes for fuels In general, alkane hydrocarbons release more energy upon combustion as they are in a more reduced state (more hydrogen atoms attached) Hence petrol (mainly octane) releases more energy than ethanol upon combustion However, alcohols have advantage of being able to be produced from renewable sources e.g. fermentation Glucose ethanol + carbon dioxide C6H12O6 2C2H5OH + 2CO2 Oxidation of alcohols Hydroxyl (-OH) group can be oxidised Oxidation depends on nature of alcohol

(primary, secondary or tertiary) Oxidising agent is potassium dichromate (VI), K2Cr2O7 Cr2O72- is reduced to Cr3+ Hence alcohol is the reducing agent Oxidation of alcohols-primary alcohols Oxidised in 2 stages: 1)Aldehyde (RCHO) 2)Carboxylic Acid (RCOOH) For ethanol: CH3CH2OH + [O] CH3CHO + H2O H H

C H C H H Ethanol C2H6O H OH H+/Cr2O72- H C

[O] NB 2H atoms removed hence oxidation H O C + H2 O H Ethanal C2H4O Oxidation of alcohols-primary alcohols- stage 2 CH3CHO + [O] CH3COOH

O H H C H H+/Cr2O72- C H [O] ethanal C2H4O

O H NB: 1 O atom added hence oxidation H C C H Ethanoic acid C2H4O2

OH Oxidation of alcohols-primary alcoholscontrolling the degree of oxidation How do we control the reaction so that the aldehyde is formed instead of the carboxylic acid or vice versa? If the aldehyde is the desired product: Heat reaction mixture in excess alcohol with distillation apparatus (to distill off aldehyde as its being formed) If carboxylic acid is the desired product: Heat reaction mixture under reflux for a longer period of time with excess oxidizing agent(a vertical condenser to ensure the aldehyde drops back into reaction vessel for further oxidation to the carboxylic acid) Distillation

Reflux Summary of oxidation of primary alcohols In general: O H R C H Primary alcohol OH

H+/Cr2O72- R C [O] O H+/Cr2O72- H aldehyde [O] R

C OH Carboxylic acid Oxidation of secondary alcohols Only 1 product formed (ketone) as there is only 1 oxidisable hydrogen attached to the secondary carbon e.g. with propan-2-ol: CH3CH(OH)CH3 + [O] CH3COCH3 + H2O O H CH3 C

OH H+/Cr2O72- CH3 Propan-2-ol (C3H8O) [O] CH3 2 H atoms lost hence oxidation C CH3

Propanone (C3H6O) H R C O OH H+/Cr2O72[O] R C R Secondary

alcohol Ketone R Oxidation of tertiary alcohols Cannot be oxidised as there is no hydrogen atom attached to the hydroxyl carbon. E.g. for 2-methylpropan-2-ol CH3 H /Cr2O + CH3

C OH CH3 27 [O] No colour change Cr2O72- stays orange No oxidation possible as no H atom on C atom bonded to alcohol group NB: tertiary alcohols can only be oxidised under harsh conditions where the carbon skeleton is broken

Tests to distinguish between aldehydes & ketones All have carbonyl (C=O) group To distinguish aldehydes/ketones from other groups: 1) Aldehydes & ketones both form orange precipitate with 2,4- dinitrophenylhydrazine 2) precipitate can be recrystallised & its melting point found (the melting point of the crystals can be used to identify the particular aldehyde/ketone) To distinguish aldehydes from ketones: 1)Treat the sample with Fehlings solution (alkaline copper(II) sulfate) or Tollens reagent (silver nitrate in ammonia) 2) Only aldehydes react with them; the aldehyde is oxidsed to the carboxylic acid as an orange-brown precipitate of copper(I) oxide is formed with Fehlings solution & a silver mirror is formed with Tollens reagent as metallic silver is deposited on the side of the test tube 3) Nothing happens with ketones

Fehlings solution- the test tube on the right has aldehyde as an orange precipitate is formed Tollens reagent- the test tube on the left has aldehyde as a silver mirror is formed Haloalkanes General formula: CnH2n+1X (X=halogen atom, F, Cl, Br, I) Are very useful as can be used to synthesize an array of other organic molecules Usually oily liquids

Halogenoalkanes - substitution reactions X replaced by another group C-X bond reactive due to the polarity difference (X is more electronegative than C) Carbon atom attached to halogen has partial positive charge (electron deficient) & is prone to attack by electron-rich species (nucleophiles) Nucleophiles Nucleo + phile Nucleus loving A species (molecule/anion) which has a lone pair of electrons which can be donated to an electron-deficient centre in an organic

molecule to form a coordinate (dative covalent) bond Examples include, -OH, H2O, NH3 Mechanisms for nucleophilic substitution The substitution of an group/group of atoms with a nucleophile as the attacking species; can occur via an SN1 (subsitution nucleophic unimolecular) or SN2 (subsitution nucleophic bimolecular) mechanism Mechanism depends on nature of haloalkane (primary, secondary, tertiary) Primary haloalkanes- mechanisms for nucleophilic substitution Take the following reaction:

CH3CH2Br(aq) + OH-(aq) CH3CH2OH(aq) + Br-(aq) Rate equation found to be rate = k[CH3CH2Br(aq)] [OH-(aq)] Hence 2 species are involved in the rate-determining step, so its bimolecular, hence SN2 In the transition state, the C-Br bond is broken at the same time as the C-O bond is being formed Hydoxide ion (nucelophile) & bromoethane Transition state Ethanol & leaving group

(Br-) Tertiary haloalkanes- mechanisms for nucleophilic substitution Take the following reaction: CH3C(CH3)2Br(aq) + OH-(aq) CH3C(CH3)2OH(aq) + Br-(aq) 2-bromo-2-methylpropane Rate = k[CH3C(CH3)2Br(aq)] Hence its unimolecular as only 1 species involved in rate determining step SN1 mechanism Different mechanism caused by steric bulk by 3 methyl groupsthe nucelophile (OH-) cannot approach the electron-deficient carbon atom SN1 mechanism Step 1 (rate-determining step): Br- departs as the leaving group; heterolytic fission

carbocation Step 2: nucleophile attacks carbocation intermediate Some definitions Carbocation- an organic ion with a positive charge on an electron-deficient carbon atom Heterolytic fission- the breaking of a covalent bond so that 1 of the atoms/groups takes both of the bonding electrons & becomes negatively charged, leaving the other atom/group positively charged Compare with homolytic fission (met earlier during the free radical mechanism of alkanes) Homolytic fission- the breaking of a covalent bond so that 1 electron from the bond is left on each atom, resulting in the formation of 2 free radicals

Other factors which favour SN1 With tertiary haloalkanes, the carbocation intermediate is stabilised by the positive inductive effect of the 3 alkyl groups (alkyl groups are have an electron-donating effect) which help to reduce the positive charge on the positive carbon e.g.: Tertiary > Secondary > Primary Nuceleophilic substitution of secondary haloalkanes & relative reactivity Go via a mixture of SN1 & SN2 mechanisms, depending on the reaction conditions, or some intermediate mechanism

Relative reactivity of different haloalkanes depends on the strength of the C-X bond Strength: C-F > C-Cl > C-Br > C-I Reactivity: C-I > C-Br > C-Cl > C-F Reaction pathways In organic chemistry, usually a desired product cannot be made from available starting materials (reactants) in a single step; hence the need for reaction pathways The production of new organic compounds from raw starting materials is called organic synthesis/synthetic organic chemistry dihaloalka ne

alkane M1- free radical subsitution 1 haloalkan e 1 trihaloalkane tetrahaloalkane 2 2 alkene

3 Poly(alken e) 4 Carboxylic acid M2-nucleophiliic subsitution alcohol 4 ketone

4 aldehyde Key: 1- subsitution halogenation 2- addition halogenation 3- polymerisation 4- oxidation NB M1 & M2 mechanism required Reaction pathway puzzles How can butanone be synthesised using 2- bromobutane as one of the starting

materials? Answer: Reflux with NaOH(aq) 2-bromobutane butanone Reflux with H+/Cr2O72- butan-2-ol Reaction pathway puzzles 2 How can ethanoic acid be synthesised using ethene as one of the starting materials? Answer: Conc. H2SO4/H2O

ethene ethanoic acid Reflux with H+/Cr2O72- ethanol

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