organic mechanisms

7/16/2019 Organic Mechanisms

1/36

Original slideprepared for the

Free Radical Substitution

AQA organic reaction mechanisms

Click a box below to go to the mechanism

Electrophilic Addition

Nucleophilic Substitution

Elimination of HX from haloalkanes

Elimination in alcohols (mechanism no longer required)

Electrophilic Substitution

Nucleophilic Addition

Nucleophilic Addition Elimination

Friedel-Crafts Alkylation Acylation

Click

here foradvice

AS

A2


2/36


CH4 + Cl2 CH3Cl + HCl

Overall reaction equation

Conditions

ultra violet lightexcess methane

i.e. homolytic breaking of covalent bonds

to reduce further substitution

Free radical substitution

chlorination of methane


3/36


CH4 + Cl CH3 + HCl

Cl2 Cl + Cl

CH3 + Cl2 CH3Cl + Cl

CH3ClCH3 + Cl

initiation step

twopropagation

steps

termination step

ultra-violet

CH3CH3CH3 + CH3 minortermination step

Free radical substitution mechanism


4/36


CH3Cl + Cl2 CH2Cl2 + HCl

Overall reaction equations

Conditions ultra-violet light

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

excess chlorine

Further free radical substitutions


5/36


Electrophilic addition

CH3CH=CH2+ Br2

CH3CHBrCH2Br

bromine with propene

hydrogen bromide with but-2-eneCH3CH=CHCH3+ HBr

CH3CH2CHBrCH3

2-bromobutane

1,2-dibromopropane

concentrated sulphuric acid with but-2-ene

CH3CH=CHCH3+ HOSO3H CH3CH2CH(OSO3H)CH3

2-butylhydrogensulphate

mechanism

mechanism

mech


6/36


Br

Br

Br

Br

Electrophilic addition mechanism

CH3

H H

H

CC

+

-

CH3

H H

HCC

Br

+

Br-

carbocation

CH3

H H

HCC

Br Br1,2-dibromopropane

bromine with propene reaction equation


7/36



CH3

H CH3

H

CC

CH3

H H

CH3CC

H

+

carbocation

CH3

H H

CH3CC

Br H

2-bromobutane

hydrogen bromide with trans but-2-ene

-

+

Br

H

reaction equation

Br-


8/36


CH3

H

CH3

H

CC

OSO3H

H+

-

OSO3H-

carbocation

2-butylhydrogensulphate

CH3

H H

CC

H

+CH3

CH3H H

CC

H OSO3H

CH3


concentrated H2SO4 with cis but-2-ene

reaction equation


9/36


hydroxide ion with bromoethane

ethanol

CH3CH2Br+ OH- CH3CH2OH + Br

-(aqueous)

Nucleophilic substitution

propanenitrile

CH3CH2I + CN- (ethanolic) CH3CH2CN + I

-

cyanide ion with iodoethane

ammonia with bromoethane

ethylamine

CH3CH2Br + NH3 CH3CH2NH22 + NH4

+Br-

mechanism

mechanism

mechanism


10/36


+ -CH3

H

BrC

H

-OH

CH3

H

OHC

H Br-

hydroxide ion with bromoethane

Nucleophilic substitution mechanism

ethanol

reaction equation


11/36


+ -CH3

H

BrC

H

CN-

CH3

H

CNC

H Br-

cyanide ion with iodoethane


propanenitrile

reaction equation


12/36


ammonia with bromoethane


ethylamine

+ -CH3

H

BrC

H

Br-

H

CH3

H

NH2C

H

+

CH3H

NH2C

H

NH3

NH4+Br-

reaction equation

NH3


13/36


Elimination of HBr from 2-bromopropane

CH3

H H

H

CC

OH-

CH3

H H

HCC

Br H

propene

H OHBr -

CH3CHBrCH3 + OH- CH3CH=CH2 + H2O + Br

-(in ethanol)

acting as a base

Elimination of HX from haloalkanes


14/36


elimination

+ OH-

RCH=CH2 + H2O + X-

(ethanol)

nucleophilic substitutionalcohol

+ OH-

RCH3CH2OH + X-

(aqueous)

RCH2CH2X

alkene

hydroxide acts as a base

hydroxide acts as a nucleophile

Haloalkanes and hydroxide ions


15/36


Elimination of H2O from propan-1-ol

CH3CH2CH2OH CH3CH=CH2 + H2O

Elimination of water from alcohols

Heat with concentrated H2SO4 orconcentrated H3PO4


16/36


CH3

H H

H

CCpropene

H OH

H+

CH3H H

OHCC

H H

CH3

H H

OHCC

H H

H

+

CH3

H H

CC

H H

+ H OH

H+

protonated alcohol

carbocation

Elimination mechanism water from propan-1-ol

reaction equation


17/36


Electrophilic Substitution

Nitration of benzene

C6H6 + HNO3 C6H5NO2 + H2O

Conditions / Reagents

concentrated HNO3 and concentrated H2SO4

50oC

mechanism


18/36


electrophilic substitution mechanism (nitration)

2. Electrophilic attack on benzene

NO2

NO2+

3. Forming the product

+

NO2

H

the nitronium ion

HNO3 + H2SO4 HSO4- + NO2 + H2O

1. Formation of NO2

+

reaction equation

+

+ H+and re-forming thecatalyst (H+ reforms

H2SO4 with HSO4-

)


19/36


Friedel-Crafts alkylation

Alkylation of benzene

C6H6 + RCl C6H5R + HCl


RCl (haloakane) and anhydrous AlCl3

0 - 25oC

Where an H atom attached to an aromatic ringis replaced by a C atom

electrophilic substitution

to prevent further substitution

R = alkyl group


20/36


Alkylation example

Three steps in electrophilic substitution mechanism

1. Formation of the electrophile (a carbocation)

AlCl3

-

Cl AlCl3CH3CH2

+

With chloroethane overall reaction equation

C6H6 + CH3CH2Cl C6H5CH2CH3 + HCl

CH3CH2 Cl


21/36


CH2CH3+

Alkylation electrophilic substitution mechanism 2


CH2CH3

ethylbenzene


CH2CH3

+H

+ H+

and re-forming thecatalyst (H+ reforms AlCl3

and HCl with AlCl4-)


22/36


Friedel-Crafts acylation

Acylation of benzene

C6H6 + RCOCl C6H5COR + HCl


RCOCl (acyl chloride) and anhydrous AlCl3

50 oC

An H atom attached to an aromatic ringis replaced by a C atom where C is part of C=O

electrophilic substitution


23/36


CH3CCl

O

Acylation example

Three steps in electrophilic substitution mechanism

1. Formation of the electrophile (an acylium ion)

AlCl3-

Cl AlCl3

With ethanoyl chloride overall reaction equation

C6H6 + CH3COCl C6H5COCH3 + HCl

+

CH3C O


24/36


Acylation electrophilic substitution mechanism 2


phenylethanone


+

CH3C O

CH3C

O

+H

C

OCH3

+ H+

and re-forming thecatalyst (H+ reforms AlCl3and HCl with AlCl4

-)


25/36



Reduction of carbonyls

RCHOaldehyde

+ 2[H] RCH2OH


NaBH4 (aq)

Room temperature and pressure

primaryalcohol

RCORketone

+ 2[H] RCH(OH)R

secondary

alcohol


26/36


+

-

Nucleophilic Addition Mechanism

reduction of propanone

H+

NaBH4 is a source ofhydride ions

RCOR + 2[H] RCH(OH)R alcohol

CH3

CH3

C

O

CH3

CH3

C

OH

H

propan-2-ol

H

CH3

CH3

C

O

H

H

H+ fromH2SO4 (aq)


27/36



RCHOaldehyde

+ HCN RCH(OH)CN


HCN (aq)

Room temperature and pressure

RCORketone

+ HCN RC(OH)(CN)R

supplies H+

and the CN-

nucleophile

addition of hydrogen cyanide to carbonylsto form hydroxynitriles


28/36


Nucleophilic Addition Mechanism

hydrogen cyanide with propanone

H+

HCN (aq) is a source ofcyanide ions

CH3COCH3 + HCN CH3C(OH)(CN)CH3

CH3

CH3

C

O

CH3

CH3

C

OH

CN

H+

2-hydroxy-2-methylpropanenitrile

fromH2SO4 (aq)+

-

CN

C N

CH3

CH3

C

O

CN


29/36



4 main types of reaction mechanism to consider1. acyl chloride + water carboxylic acid2. acyl chloride + alcohol ester3. acyl chloride + ammonia acid amide4. acyl chloride + amine N-substituted amideHCl is a secondary product in all of the above

reactions.Two of the 4 mechanisms are shown on the next few

slides, but all 4 versions must be known for theexam. Mechanisms for acid anhydrides need not belearned (but equations should, giving a carboxylic

acid in place of HCl)


30/36



Acylation of water to give carboxylic acids

RCOCl + H2O RCOOH

Conditions room temperature and pressure

carboxylic acid

+ HCl

OHR C

O


31/36


Formation of ethanoic acid

+ H2O CH3COOH + HClCH3COCl

ethanoyl chloride

ethanoic acid

mechanism

OHCH3 C

O


32/36


Nucleophilic Addition Elimination Mechanism

CH3

Cl

CO+-

OH

H

CH3

Cl

C

O

OH

H

+

CH3 C

O

OH

H

+

CH3 C

O

OH

+ HCl

Cl

nucleophilic

addition

elimination

reaction equation


33/36


Acylation of primary amines to N-alkyl amides

RCOCl + RNH2 RCONHR

Conditions room temperature and pressure

N-alkylamide

+ HCl

NHRR C

O

N-propylethanamideNHCH2CH2CH3

CH3 CO


34/36


Formation of N-propyl ethanamide

from

+ CH3CH2CH2NH2

CH3CONHCH2CH2CH3 + HCl

CH3COCl

ethanoyl chloride 1-aminopropane

N-propylethanamide

mechanism


35/36


+-

Nucleophilic Addition Elimination Mechanism

CH3

Cl

C

O

NHCH2CH2CH3

H

CH3

Cl

C

O

NHCH2CH2CH3

H

+

CH3 C

O

NHCH2CH2CH3

H

+

+ HCl

CH3

C

O

NHCH2CH2CH3 Cl

nucleophilic

addition

elimination

reaction equation


36/36

Original slide

Steve Lewis for the Royal Society of Chemistry

Adapted by Mark Smith for Turton ChemistryDepartment

References

organic mechanisms

Documents

thehydroxide ion

ch3 hclcl2

theelectrophilic additionch3ch

propenehydrogen bromide

alcohols mechanism

thech3hch3hccoso3hh

ch2 br2ch3chbrch2brbromine

covalent bondsto