org chem reactions

reactions for alkanes

• combustion

• free radical substitution

combustion

• complete combustion occurs in excess oxygen and heat.

  • alkanes: CxHy + (x+y/4) O2 → xCO2 + y/2 H2O

  • alcohols: CxHyOz + (x+y/4-z/2)O2 → x(CO2) + (y/2)H2O

• incomplete combustion in limited oxygen. produces C (soot), CO and CO2

• alkanes vs alcohols

  • alcohol produces less energy: lower standard enthalpy of combustion

  • therefore gasoline more reliable

free radical substitution reagents and conditions

• alkane + halogen → halogenoalkane

• UV light/high temperature 250-400C

free radical substitution mechanism

1. initiation: homolytic fission of Cl-Cl bond → forms chlorine radicals

2. propagation: radicals consumed + regenerated

   • Cl• consumed as it removes H atom from molecule to produce H3C•, which reacts with another Cl2 molecule to produce CH3Cl and regenerate another Cl• and so on.

   • CH3Cl can undergo further substitution in the presence of excess Cl2 → to avoid, limit concentration of Cl

   • unable control which product forms, will always produce a mixture

3. termination: 2 radicals consumed (chain termination steps)

   • as reaction progresses, conc of molecules decrease, conc of radicals increase, probability of radical consuming itself increases

reactions for alkenes

• reduction

• electrophilic addition

  • addition of halogen

  • addition of hydrogen halide

  • addition of inter-halogen compounds

  • addition of water (hydration)

reduction of alkenes reagents and conditions

• alkene + H2 (g) → alkane

• Ni catalyst, heat 150C OR Pt/Pd catalyst, room temp

electrophilic addition mechanism

• pi electron cloud polarises molecule

• d+ electrophile attacks pi electrons in C=C, formation of sigma bond and carbocation intermediate

• electron-rich d- donates electron pair to electron-deficient carbocation, forms sigma bond to form stable addition product

explanation for positive inductive effect (electron donating inductive effect)

“tertiary carbocation has more alkyl groups than primary carbocation, exerts stronger positive inductive effect, disperses charge to larger extent so more stable, formed more readily.”

• no. of R groups/size of R groups increases, stronger electron donating inductive effect

  • no. of R groups is more significant

• helps disperse positive charge to a larger extent

• tertiary carbocation more stable, formed more readily

electrophilic addition of halogen to alkene reagents and conditions

• alkene + halogen → dihalogenoalkane

• CCl4 solvent, room temp

• used to distinguish alkanes and alkenes. alkene decolourises red-brown bromine.

electrophilic addition of hydrogen halide to alkene reagents and conditions

• alkene + hydrogen halide → halogenoalkane

• dry hydrogen halide, room temp

electrophilic addition of inter-halogen compounds to alkene reagents and conditions

• alkene + BrCl → dihalogenoalkane

• liquid BrCl, rtp

electrophilic addition of water to alkene (hydration) reagents and conditions

• alkene + H2O → alcohol

• lab: cold conc H2SO4 catalyst, followed by boiling with H2O

• industrial: H2O (g), heat 300C, pressure 60-70atm, conc H3PO4

reactions for arenes (aromatic compounds)

• electrophilic substitution: addition of nitrogen

• reduction of nitrobenzene

nitration of benzene (electrophilic substitution) reagents and conditions

• benzene + conc HNO3 → nitrobenzene

• conc H2SO4 catalyst, 50C

reduction of nitrobenzene

• nitrobenzene → phenylamine

• Sn (reducing agent) in conc HCl, heat under reflux followed by NaOH (aq)

electrophilic substitution mechanism

1. generate positively-charged electrophile

   • H₂SO₄ catalyst protonates hydroxyl group in HNO₃

   • N O bond breaks, forms H₂O and NO²⁺

   • overall HNO₃ + 2H₂SO₄ <=> NO₂⁺ + H₃O⁺ + 2HSO₄^-

2. benzene has high electron density, attacks electrophile NO²⁺ to form non-aromatic carbocation intermediate (SLOW STEP)

3. H-benzene bond attacks benzene carbocation to restore benzene ring’s delocalised pi electron ring system. lone pair from HSO₄^- attacks H, regenerate catalyst H₂SO₄

notes:

• benzene resonance structure makes it stable → prefers substitution, doesn’t undergo addition

• high activation energy since need to break resonance structure → needs lewis acid catalyst to generate strong electrophile

reactions for halogenoalkanes

• nucleophilic substitution

  • with OH^- → alcohol

  • with NH₃ → amine

  • with CN^- → nitrile

nucleophilic substitution reagents and conditions

formation of alcohol from halogenoalkane

• halogenoalkane + OH^- → alcohol + halogen anion

• NaOH (aq), heat under reflux

formation of amine from NH₃

• halogenoalkane + NH₃ → amine + salt (eg NH₄Cl)

• ethanol, heat

formation of nitrile from CN^-

• halogenoalkane + KCN/NaCN → nitrile + salt (eg NaCl)

• ethanol, heat under reflux

nucleophilic substitution SN1 mechanism

• tertiary halogenoalkanes (tetrahedral)

  1. polarised C-Br bond undergoes heterolytic fission → forms Br^- and trigonal planar carbocation (slow step)

  2. lone pair on nucleophile attacks carbocation, forms bond (fast step)

• equal probability for nucleophile to attack from top/bottom of trigonal planar carbocation (to become tetrahedral again). therefore product is a racemic mixture of pair of enantiomers.

• 2-step reaction, rate = k[halogenoalkane], overall order of reaction 1

nucleophilic substitution SN2 mechanism

• primary halogenoalkanes

• nucleophile backside attack d+ C atom of C-Br bond

  • nucleophile attacks from back side because electron density mostly on halogen side

  • electrons from nucleophile repel d- halogen, lengthens C-Br bond

• forms 5-membered trigonal bipyramidal transition state (negatively charged)

  • in transition state, both nucleophile and halogen are partially bonded to same C atom (represented by dotted lines)

  • note: transition state, not intermediate since only 1 step

• concerted: nucleophile backside attack + halogen leaving group at same time, therefore 1 step reaction

• 1-step reaction, rate = k[halogenoalkane][nucleophile], overall order of reaction 2

why SN1 preferred?

• in tertiary halogenoalkanes the three electron-donating R groups positive inductive effect will stabilise the C+ more

• forms a more stable carbocation intermediate, therefore preferred mechanism SN1

steric hindrance

• tertiary halogenoalkanes C+ surrounded by 3 bulky R groups

• cannot proceed by SN2 mechanism because no room to accomodate 5 bulky groups in the transition state (note: transition state, not intermediate since only 1 step)

why SN2 preferred?

• primary halogenoalkanes low steric hindrance: 2 groups are H atoms so there is room for other 3 groups

• can have 5 groups surrounding central C in transition state, prefers SN2

factors affecting rate of nucleophilic substitution

1. class of halogenoalkane → rate eq of mechanism

2. reactivity of C-X bond → bond strength affected by resonance (unreactive) / electronegativity (less negative, more reactive)

3. type of nucleophile → availability of lone pair depends on charge (more negative, stronger nucleophile) + electronegativity (less negative, stronger nucleophile)

how does mechanism affect rate of nucleophilic substitution?

• SN1 ‘transition state’ just lengthening bond vs SN2 transition state lengthen bond and increase crowdedness

• SN1 ‘transition state’ thus has lower Ea

why don’t chlorobenzene/chloroalkenes undergo nucleophilic substitution?

• p orbital of Cl overlaps with p orbitals of benzene ring / C=C

• Cl part of resonance structure, C-Cl bond strong so breaks less easily

how does type of halide leaving group affect rate of nucleophilic substitution?

• size of halogen increases, more diffused overlap of valence orbitals → bond length of C-halogen bond increases

• bond strength decreases, energy needed to break bond decreases, rate increases

• rate: (fastest) iodoalkane > bromoalkane > chloroalkane (slowest)

how does reactivity of nucleophile affect rate of nucleophilic substitution?

• anions > neutral

  • higher electron density, more reactive

• within same charge, less electronegative > more electronegative

  • less electronegative, electrons held less strongly, donate electron pair more easily, more reactive

note: ONLY FOR SN2, rate determining step of SN1 doesn’t involve Nu^-

preparation of alcohol

1. electrophilic addition of alkene

   • industrial: steam, heat with conc H₃PO₄ catalyst, 300C, 70atm

   • lab: conc H₂SO₄, heat with H₂O

2. nucleophilic substitution of halogenoalkanes

   • NaOH/KOH (aq), heat under reflux

3. reduction of aldehydes, ketones, carboxylic acids (opposite of oxidation of alcohol)

   • aldehydes and carboxylic acids reduced to form primary alcohols, ketones reduced to form secondary alcohols

   • in order of reducing strength:

     • LiAlH₄ in dry ether, followed by addition of dilute acid

     • NaBH₄ in dry ethanol followed by addition of dilute acid (only for aldehydes and ketones)

     • heating with hydrogen gas and nickel catalyst (only for aldehydes and ketones)

reduction of aldehydes

• aldehyde + 2H → primary alcohol

• C=O pi bond breaks, 2H added across

• in order of reducing strength:

  • LiAH₄ in dry ether, followed by addition of dilute acid

  • NaBH₄ in dry ether followed by addition of dilute acid

  • heating with hydrogen gas and nickel catalyst

reduction of ketones

• ketone + 2H → secondary alcohol

• C=O pi bond breaks, 2H added across

• in order of reducing strength:

  • LiAH₄ in dry ether, followed by addition of dilute acid

  • NaBH₄ in dry ether followed by addition of dilute acid

  • heating with hydrogen gas and nickel catalyst

reduction of carboxylic acids

• carboxylic acid + H → aldehyde

• aldehyde + H → primary alcohol

• C-OH bond breaks, -OH lost as water by bonding with H, replaced by C-H bond (forms aldehyde)

• C=O pi bond breaks, add 2H across (forms primary alcohol)

• reagents and conditions

  • LiAH₄ in dry ether, followed by addition of dilute acid

  • CANNOT use NaBH₄ or hydrogen gas because resonance makes carboxylic acid stable

why NaBH₄ cannot reduce carboxylic acid but LiAlH₄ can?

• bond length of Li-Al > Na-B

• bond strength decrease, energy needed decrease, reactivity increase

• carboxylic acid resonance makes it stable, need stronger reducing agent

• therefore LiAlH₄ stronger reducing agent than NaBH₄, can reduce carboxylic acids, aldehydes and ketones

H₂ vs NaBH₄/LiAlH₄

• H₂ is non-polar RA, so weaker. can reduce C=C and aldehyde/ketones

• NaBH₄/LiAlH₄ are polar (hydride, H^-) so stronger, BUT electron-rich C=C repels nucleophilic RA. aldehyde, ketone and carboxylic acid are polar, d+ C reacts with nucleophilic RA. (but acid only with LiAlH₄)

reactions for alcohols

• combustion

• oxidation

• condensation (esterification)

oxidation of alcohol

• primary alcohols oxidised to aldehydes under controlled conditions, aldehydes further oxidised to carboxylic acids

  • has 2 alpha H → 2 step oxidation

• secondary alcohols oxidised to ketones

  • has 1 alpha H → 1 step oxidation

• tertiary alcohols do not undergo oxidation

  • no alpha H, cannot oxidise

formation of aldehyde

• oxidation of primary alcohol

• acidified (using H2SO4) weak OA: K2Cr2O7

• heat with immediate distillation

  • so that doesn’t oxidise to become carboxylic acid

formation of carboxylic acid

• 2-step oxidation of primary alcohol

• acidified (using H2SO4) excess strong OA: KMnO4 or K2Cr2O7

• heat under reflux

  • prevent loss of volatile compound which would evaporate, by condensing it back into the mixture

alcohol oxidation mechanism

primary alcohols: 2 alpha hydrogen (2 H on the C attached to the -OH), 2 step oxidation

1. first alpha H and H from -OH are lost as water by bonding with O (from OA)

   C forms pi bond with O (C-O → C=O)

2. second alpha C-H bond is oxidised by adding O (from OA)

secondary alcohols: 1 alpha hydrogen, 1 step oxidation

1. alpha H and H from -OH are lost as water by bonding with O (C-O → C=O)

tertiary alcohols: no alpha hydrogen, no oxidation

condensation of alcohol

• alcohol + carboxylic acid <=> ester + water

• conditions

  • conc H2SO4 → catalyst + dehydrating agent to remove water and favour forward reaction

  • heat under reflux

• H from alcohol and OH from ethanol lost to form H2O, ester bond formed between RO- and ROC-

hydrolysis of ester

• reverse of condensation of alcohols

• ester + water <=> alcohol + carboxylic acid

• conditions

  • H2SO4 (aq) → aqueous to provide H2O

  • heat under reflux → prevent loss of volatile compounds

• H2O breaks ester bond, H attaches to RO- to form alcohol, OH attaches to ROC- to form RCOOR ester

transesterification

• replace alcohol (-OR) of ester (RCOOR) with a different alcohol

  • RCOOR + R’OH <=> RCOOR’ + ROH

  • reversible reaction, excess R’OH alcohol used to shift POE towards products

• conditions: strong acid/base catalyst (H2SO4, NaOH etc)

• application: forming biodesel from plant oils

  • vegetable oils cannot be used directly in engine because too viscous + solidify at low temperature

  • vegetable oils: formed from 3 fatty acids (separate carboxylic acids) reacting with glycerol (triol) → triglycerides

  • transesterification: each fatty acid molecule bonds with methanol

    • methanol react with NaOH to form CH3ONa which is then reacted with the vegetable oils

    • since reversible reaction, excess alcohol used

    • biodiesel must be separated from glycerol (denser and sinks) and purified before use

addition polymerisation

• alkene monomers, pi bond breaks to form repeating unit

• no by-product, 100% atom economy

• ethene and propene obtained from cracking

• poly__ (monomer name)

  • polyethene

  • polychloroethene / polyvinylchloride (PVC) → ethene with 1 H replaced by Cl

  • polyvinylalcohol (PVA) → ethene with H replaced by OH

  • polytetrafluoroethene (PTFE) → ethene with 4 F instead of H

  • polysytrene (PS) → ethene with 1 H replaced by benzene ring

  • polypropene (PP) → propene

condensation polymerisation for polyesters

• two monomers react to form polyesters and water/HCl

• R’OH + RCOOH <=> RCOOR’ + H2O (note formation of ester bond)

• conditions: conc H2SO4 catalyst (and dehydrating agent), heat under reflux

• two cases

  1. 2 monomers: dicarboxylic acid + diol

  2. 1 monomer with 2 diff functional groups on each side

     note: if n is a even number, both cases would look the same, cannot tell what the monomers are (puzzle analogy)

condensation polymerisation for polyamides

• two monomers react to form polyesters and water/HCl

• two cases

  1. 2 monomers: diamine + dicarboxylic acid → polyamide + (2n-1)H2O

  • n is no. of repeating units

  • nylon 6,6 → made up of 2 6-C carbon chains

  • note: if n is a even number, both cases would look the same, cannot tell what the monomers are (puzzle analogy)

  2. 1 monomer with 2 diff functional groups on each side

• conditions: high temperature, high pressure, presence of catalyst

• intermolecular interaction between polymer chains: H atom on N-H forms hydrogen bond with C=O on other chain

nitrobenzene → phenylamine 2-stage reaction

1. nitrobenzene reduced with Sn in acidic environment to form intermediate ion and Sn²⁺ ions

2. intermediate ion converted to phenylamine in the presence of OH^-