Organic Chemistry (AS + A-Level)

Alkanes

Saturated hydrocarbons

General formula = C_nH_2n+2

Fractional distillation of alkanes

Alkanes are found in crude oil (mixture of different lengths of hydrocarbons)

• Column has temp gradient: cooler at top, hotter at bottom

  • Longer chain hydrocarbons condense

Uses of crude oil

Gas - stove

Kerosene - jet fuel

Bitumen - roofing

Diesel oil, petrol, fuel oil

Purpose of cracking

Heavier fractions can be cracked into higher demand, lighter fractions

Thermal cracking conditions + common products

Conditions: high temperature (1000^o C) + high pressure (70 atm)

Products are usually alkenes (used to make polymers)

Catalytic cracking conditions + common products

Conditions: high temperature (40^o C) + slight pressure used with zeolite catalyst

• Zeolite lowers temp + pressure needed so lower costs and faster process

Products are usually aromatic hydrocarbons (useful for fuels in vehicles)

Complete combustion of alkanes

Burn in oxygen to form CO₂ and H₂O

Alkanes are good fuels as most burn readily to produce large amounts of energy

Used to power vehicles + most electricity

Incomplete combustion of alkanes

Burn in limited O₂ supply to form CO (carbon monoxide) and soot

• CO is poisonous: binds to haemoglobin in blood which prevents O₂ binding, can be removed using catalytic converter

• Soot causes breathing problems

Global warming

Burning fossil fuels produces CO₂ (greenhouse gas)

• CO₂ absorbs IR from sun but emits some back to earth

Removal of sulfur dioxide from flue gases

Using CaO: CaO (s) + 2H₂O (l) + SO₂ (g) → CaSO₃ (s) + 2H₂O (l)

Using CaCO₃: CaCO₃ (s) + 2H₂O (l) + SO₂ (g) → CaSO₃ (s) + 2H₂O (l) + CO₂ (g)

Photochemical smog

Oxides of nitrogen made from N and O₂ in air combining under high pressure + temp e.g., car engines (catalytic converts can reduce NO_x going into atmosphere)

Harms respiratory system + toxic

Acid rain

Burning fossil fuels releases SO₂

• Reacts w H₂O to form H₂SO₄ which falls as acid rain: damages plants, kills fish, causes erosion

Wet scrubbing can remove SO₂ by neutralising with CaCO₃

Initiation

Radicals are produced normally using UV light

Bond breaks, producing 2 radicals

E.g., Cl-Cl → Cl•

Propogation

Radical reacts with non-radical, new radicals are created which react with more non-radicals

E.g., CH₃CH₃ + Cl• → •CH₂CH₃ + HCl

Termination

When 2 radicals react, they form non-radical molecule which ends chain reaction

E.g., •CH₂CH₃ + Cl• → ClCH₂CH₃

Halogenoalkanes

Alkanes with one or more halogens attached to it

BP trends of halogenoalkanes

Increases down the group

• More electrons = stronger vdw forces so more energy needed to overcome

Bond polarity of halogenoalkanes

Have a polar bond + attacked by nucleophiles

• Halogens are more electronegative than carbon so pull electrons towards themselves

Nucleophile

Substance that is an electron pair donor e.g., NH₃, OH^-, CN^-

Halogenoalkane reaction with hydroxide ions (ns)

Via nucleophilic substitution in warm aqueous NaOH + under reflux

1) Nucleophile attacks δ+ carbon

2) Nucleophile replaced halogen

R-X + NaOH → ROH + NaX

Halogenoalkane reaction with cyanide ions

Via nucleophilic substitution in warm ethanolic KCN + under reflux, creeates nitriles

1) Nucleophile attacks δ+ carbon

2) Nucleophile replaces halogen

R-X + KCN → RCN + KX

Halogenoalkane reaction with ammonia

Via nucleophilic substitution in warm ethanolic NH₃ (excess)

1) Ammonia attacks δ+ carbon

2) Ammonia replaces halogen, forming intermediate

3) Another ammonia acts as base by reacting w hydrogen

4) Amine produced and ammonium ion produced

Halogenoalkane reactivity

Increases down the group

• Bond strength/bond enthalpy determines reactivity, not bond polarity

Halogenoalkane reaction with hydroxide ions (e)

Via elimination in warm ethanolic NaOH under reflux

1) OH^- attacks hydrogen on carbon adjacted to carbon with halogen

2) OH^- acts as a base, forming water

3) Electrons in bond move to form double bond between two carbons

4) C-X breaks, both electrons move from bond to halogen

Chlorofluorocarbons (CFCs)

Molecules that have had all their hydrogens replaced by chlorine and fluorine

Effect of CFCs on ozone (O₃)

Breaks down O₃ in atmosphere

• C-Cl bond broken by UV radiation - radicals formed, catalyse breakdown of ozone

• C-Cl bonds are broken easiest by UV as they have lowest bond enthalpy

Equations for CFCs destroying ozone

1) Initiation

CCl₃F → CCl₂F + Cl•

2) Propogation

Cl• + O₃ → O₂ + ClO•

ClO• + O₃ → 2O₂ + Cl•

3) Termination

Cl• + Cl• → Cl₂

Restricting use of CFCs

CFCs are stable, unreactive, non-toxic + were refrigerants but destroy ozone

HFCs used instead as they do not have chlorine

Alkenes

Unsaturated hydrocarbons, general formula = C_nH_2n

Cycloalkenes

Have 2 fewer hydrogens than alkenes, general formula = C_nH_2n-2

Alkenes electrophilic addition reactions

Alkenes are attacked by electrophiles due to double bond

• Double bond has high electron density

Electrophile

Electron pair acceptor e.g., NO₂⁺, H⁺, H-Br, H₂SO₄

Bromine test for alkenes

Causes color change from brown-orange to colourless

• Br is electrophile + adds to alkene to form dibromoalkene

Alkenes - addition of hydrogen halides

React with hydrogen halides to form halogenoalkanes

• Reacting with unsymmetrical alkenes forms 2 different products (amount of products depends on stability of carbocation intermediate)

• More alkyl groups bonded to carbocation = more stable intermediate

• Alkyl groups push electrons towards carbocation + stabilises it

Stability: Tertiary > Secondary > Primary

Alkenes - addition of sulfuric acid

React with cold, conc H₂SO₄ to form alkyl hydrogen sulfates

If water added, alcohol will reform

E.g., H₂C=CH₂ + H₂SO₄ → CH₃CH₂OSO₂OH

CH₃CH₂OH + H₂O → CH₃CH₂OH + H₂SO₄

Alkenes into addition polymers

Alkenes are monomers with join to form addition polymers - can be natural (proteins/rubber) or synthetic (poly(ethene))

Polyalkene properties

Most polyalkene chains are non-polar so only vdw forces

Longer chain + closer together = more vdw

Some polyalkenes have halogens (can have permanent dipole-dipole)

Plasticisers

Are added to polymers to increase flexibility - slide between polymer chains, pushing them apart, weakening intermolecular forces so chains can slide over each other + bend

PVC and plasticisers

PVC made from long, closely-packed polymer chains that are hard but brittle (used in drain pipes)

PVC w plasticisers is more flexible (used for electrical insulation + clothing)

Alcohols

Have functional group -OH (hydroxyl), general formula = C_nH_2n+1OH

Dehydration of alcohols/hydration of alkenes

e.g., ethanol → ethene + water (use of acid catalyst)

Alkenes can be made from ethanols sustainably if alcohol has been made via fermentation of glucose from plants

Elimination reactions of alcohols

Dehydration of non-primary alcohols can lead to 2 different alkenes - C=C can be formed on either side of carbon with OH

1) Lone pair on O will attach to H+ from acid catalyst

2) Intermediate formed has a +ve charge on O - O pulls electrons in C-O bond strongly to break bond, leaving unstable carbocation intermediate

3) Carbocation loses H+ - electrons in C-H move to form C=C bond

Making alcohols from hydration of alkenes

Steam and acid catalyst used

E.g., ethanol:

1) React steam + ethene with phosphoric acid catalyst

2) Temp of 300^oC and 60 atm

Making alcohols from fermentation

Making ethanol via fermentation uses renewable source of glucose from plants

Uses yeast in anaerobic conditions - reaction is exothermic

Requires little equipment + uses renewable resources so is cheap

Biofuels

Made from dead biological matter

• Ethanol is being used as fuel in countries with good supply of sugar cane

• Sugar is fermented to produce alcohol

Advantages of biofuels

Renewable so more sustainable than crude oil

Produce CO₂ when burnt but classed as carbon neutral because carbon dioxide absorbed by sugar cane when growing

Disadvantages of biofuels

Expensive to convert existing petrol engines to take fuels w higher conc of ethanol

Land could be used to grow food rather than make fuel, could cause food shortages in countries growing sugar canes

Process of fermentation to make alcohol

1) Plants taken in CO₂ + make glucose via photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

2) During fermentation, ethanol is produced

C₆H₁₂O₆ → 2CO₂ + 2C₂H₅OH

3) Combustion of ethanol

2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O

Oxidation of alcohols

Used acidified potassium dichromate, K₂Cr₂O₇

Primary alcohols oxidise to aldehydes then carboxylic acids

Secondary alcohols oxidise to ketones

Tertiary alcohols cannot be oxidised

Mass spectrometry

Used to find relative molecular mass of compound

• Peaks show fragments of original molecule

• Last peak = M+1 peak (same as relative molecular mass of molecule)

• m/z is mass divided by charge

Infrared spectroscopy

Uses infrared radiation to increase vibrational energy of covalent bonds in sample

• Frequency depends on atoms either side of bond + position of bond in molecule

Purpose of fingerprint region in IR spectroscopy

Can compare fingerprint region against known library of spectra to identiy molecule

Extra peaks in fingerprint region indicates impurities in sample

Infrared and global warming

1) EM radiation from sun reaches earth + is absorbed by land and sea - some is re-emitted as infrared

2) Greenhouse gases absorb radiation + re-emit this back towards earth - covalent bonds absorb radiation

Optical isomerism

Form of stereoisomerism - have the same structural formula but different arrangement of atoms in space

Optical isomers

Are mirror images of each other + have chiral carbon atom

Chiral molecule

4 different groups attached to C atom; can be arranged in two different ways (enantiomers)

Enantiomers

Mirror images to each other + non-superimposable (do not overlap)

Identifying optically active isomers

Use plane-polarised light

• One enantiomer rotates light clockwise, other rotates light anticlockwise

Racemic mixture/racemates

Equal amounts of each enantiomer - does not rotate plane polarised light

Molecules with planar profiles (e.g., double bonds in C=C or C=O) can make racemic products

• Planar nature so even chance of nucleophile attacking from top or bottom so 50/50 mixture

Aldehydes

Have carbonyl group (C=O) on an end carbon, ends with -al

E.g., propanal CH₃CH₂CHO

Ketones

Have carbonyl group (C=O) on inner carbon, ends with -one

E.g., propanone CH₃COCH₃

Oxidation of aldehydes and ketones

Aldehydes are readily oxidised into carboxylic acids

Ketones do not oxidise

Testing for aldehydes + ketones - Tollen’s

1) Add few drops of NaOH to silver nitrate solution (pale brown ppt formed)

2) Add few drops of dilute ammonia until ppt dissolves

Aldehydes: silver mirror formed

Ketones: no silver ppt

Testing for aldehydes + ketones - Fehling’s

Aldehydes: goes from blue solution to brick red ppt

Ketones: remains blue

Reduction of aldehydes and ketones

Use NaBH₄ in aqueous solution

Aldehydes: reduce to primary alcohol

Ketones: reduce to secondary alcohol

Nucleophilic addition with aldehydes and ketones

1) Nu attacks carbonyl carbon to form a -ve charged intermediate which quickly reacts with proton

Aldehydes + ketones with KCN

Produces hydroxynitriles via nucleophilic addition

1) CN^- ion attacks C=O and adds on to make hydroxynitrile (with OH and CN group)

Aldehyde: RCHO + KCN + H⁺ → RCH(OH)CN + K⁺

Ketone: RCOR’ + KCN + H⁺ → RCR(OH)CN + K⁺

Using KCN

Risks: irritant, forms toxic gas (HCN)

Ways to reduce risk: wear lab coat, wear safety goggles, use fume cupboard, wear gloves

Carboxylic acids

Have carboxyl (-COOH) functional group; contains carbonyl (C=O) and hydroxyl (O-H) group, general formula = C_nH_2n+1COOH (COOH always on end of molecule)

Reactions of carboxylic acids

Weak acids - react with carbonates to form CO₂

• Partially dissociate to form H⁺ ion and carboxylate ion

React with carbonates to form salt, carbon dioxide gas and water

E.g., 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Esters

General formula RCOOR’, formed from:

Carboxylic acid + alcohol (acid catalyst) → ester + water

Acid anhydride + alcohol → ester + carboxylic acid

Naming of esters

First half = alcohol, second part = acid

E.g., methanol + ethanoic acid → methyl ethanoate

Uses of esters

1) Perfumes + food flavorings - some have sweet smells

2) Solvents → are polar so other polar compounds can dissolve, have low bps + evaporate easily so valuable for glue

3) Plasticisers

Can make plastics more flexible during polymerisation, penetrates polymer chains + increases distance between them

Acid hydrolysis of ester

Ester → Carboxylic Acid + Alcohol

Under reflux, heat, dilute acid (HCl, H₂SO₄)

Equilibrium established/does not go to completion

Base hydrolysis of ester

Ester → Carboxylate salt + alcohol

Under reflux, heat, dilute alkali (NaOH)

Reaction is irreversible/goes to completion

Fats and oils

Glycerol (alcohol) + fatty acids (carboxylic acids) → ester (makes fats and oils)

Glycerol (propane-1,2,3-triol) reacts w long chain fatty acids which can be saturated and unsaturated

Vegetable oils v animal fats

Vegetable oils: unsaturated hydrocarbon chains, not straight so cannot pack closely together so low vdw forces

Animal fats: saturated, straight so can pack closely so high vdw forces

Soap from animal fats/vegetable oils

Animals fats/vegetable oils can be hydrolysed by heating with NaOH to form soap

Fat + NaOH → glycerol + crude soap (saponification)

Biodiesel from vegetable oils

Biodiesel → mixture of fatty acids made from methyl esters + can be made by rapeseed oil

Vegetable oils can be converted to biodiesel by reaction with methanol + using KOH as catalyst

Triglyceride + methanol → fatty acid methyl esters + glycerol

Acyl chlorides

Have functional group (-CO-Cl) which contains acyl group (COCl), always at end of molecule

Reaction of acyl chloride with water

Acyl chloride + water → carboxylic acid

E.g., ethanoyl chloride with water

CH₃COCl + H₂O → CH₃COOH + HCl

Vigorous reaction, white misty fumes of HCl produced

Reaction of acyl chloride with ammonia

Acyl chloride + ammonia → amide

E.g., ethanoyl chloride with ammonia

CH₃COCl + NH₃ → CH₃CONH₂ + HCl

Vigorous reaction, white misty fumes of HCl gas

Reaction of acyl chloride with alcohol

Acyl chloride + alcohol → ester

E.g., ethanoyl chloride with methanol

CH₃COCl + CH₃OH → CH₃COOCH₃ + HCl

Vigorous reaction, white misty fumes of HCl

Reaction of acyl chloride with primary amine

Acyl chloride + primary amine → n-substituted amide

E.g., ethanoyl chloride with primary amine

CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl

Vigorous reaction, white misty fumes of HCl

Acid anhydrides

Molecules made from 2 carboxylic acids that are the same

e.g., ethanoic acid + ethanoic acid → ethanoic anhydride

Reaction of acid anhydride with water

Acid anhydride + water → carboxylic acids

E.g., ethanoic anhydride with water

C₄H₆O₃ + H₂O → CH₃COOH

Reaction of acid anhydride with ammonia

Acid anhydride + ammonia → amide

E.g., ethanoic anhydride with ammonia

C₄H₆O₃ + NH₃ → CH₃CONH₂ + CH₃COOH

Reaction of acid anhydride with alcohol

Acid anhydride + alcohol → ester

E.g., ethanoic anhydride with methanol

C₄H₆O₃ + CH₃OH → CH₃COOH₃ + CH₃OOH

Reaction of acid anhydride with primary amine

Acid anhydride + primary amine → n-substituted amides

E.g., ethanoic anhydride with primary amine

C₄H₆O₃ + CH₃NH₂ → CH₃CONHCH₃ + CH₃COOH

Acyl chlorides and nucleophilic addition-elimination

Acyl chlorides have strong δ+ on carbon which can be attacked by nucleophiles (due to Cl being electronegative)

1) Addition of nucleophile across C=O bond

2) Elimination of small molecule such as HCl or H₂O

Nucleophiles such as NH₃, H₂O, CH₃NH₂

Making aspirin

Salicylic acid + ethanoic anhydride → aspirin + ethanoic acid

Why use ethanoic anhydride than ethanoyl chloride for aspirin production

Cheaper, less corrosive, safer, produces less toxic by product, does not react with water readily

Benzene

Cyclic, planar molecule with formula C₆H₆

Structure of benzene

Each C is bonded to 2 other Cs + 1 H

Final lone electron is in p-orbital which sticks out above + below planar ring

• Lone electrons in p-orbitals combine to form delocalised ring of electrons

All C-C bonds in molecule are same due to delocalised electron structure

Stability of benzene

Benzene is more stable than theoretical alternative cyclohexa-1,3,5-triene

Benzene (-360 kJ mol) has lower enthalpy change than cyclohexa-1,3,5-triene (-120 kJ mol) so more energy required to break bonds + more energy released to form bonds

Arene

Aromatic compound that contains benzene ring

Named in 2 ways:

1) Benzene at end e.g., bromobenzene

2) Phenyl, C₆H₅, as functional group e.g., phenylamine

Reactions of arenes

Benzene has high electron density due to delocalised ring of electrons (attractive to electrophiles)

Benzene is stable so does not undergo electrophilic addition, it undergoes electrophilic substitution so ring of electron is not disturbed

Friedel-Crafts acylation

Acyl group (RCO-) is added onto benzene so structure becomes weaker so easier to modify + make into useful products

• Electrophile must have very strong +ve charge to add onto benzene ring

• AlCl₃ (halogen carrier) is used as a catalyst to produce stronger electrophile from acyl group

Nitration of benzene

Nitrobenzene made from heating benzene w conc HNO₃ and H₂SO₄ but powerful electrophile needed first

• Used in dyes + pharmaceuticals

• Used in TNT

1) HNO₃ + H₂SO₄ → H₂NO₃⁺ + HSO₄^- + NO₂⁺ + HSO₄^-

2) Electron ring attracted to NO₂⁺ then replaces H with NO₂

3) HSO₄^- + H⁺ → H₂SO₄

Amine

Derived from ammonia molecules + contain a nitrogen atom where hydrogens are replaced with an organic group

Primary amine = 1 organic group

Secondary amine = 2 organic groups

Tertiary amine = 3 organic groups

Quarternary amine = 4 organic groups (exists as +ve salt)