LECTURE NOTE SK025 (ORGANIC) KMKt

SN2 MECHANISM

❷ Second–order reaction

Rate = k[R—X][:Nu–]

EXAMPLE :

CH3—Cl + OH– CH3—OH + Cl–

Rate = k[CH3Cl][OH–]

❑ depend on [CH3Cl] and [OH–]
❑ second order overall

SN2 MECHANISM

❸ One–step mechanism

❑ Has a transition state
❑ No carbocation rearrangement

Reaction: CH3Cl + HO– CH3OH + Cl–

SN2 MECHANISM

❑ All SN2 reactions proceed with backside
attack, resulting in inversion of
configuration at a stereogenic center

Ref: Wade, 7th Edition, pg 241

H H
H H

- δ+ δ– HO C + Cl–

HO C Cl H

H

Backside attack literally attack
from the C atom inside out, like
an umbrella caught by the wind

STERIC EFFECT

❑ SN2 reactions are affected by steric factors
(steric effect).

Steric effect :
Is an effect on relative rates caused by the space -
filling properties of parts of molecules attached at or
near to the reacting site.

The reactivity on SN2 reaction depends on the
size of atoms or groups attached to the C – X.
Larger no. of alkyl groups, shielded the carbon
atom in the C-X from attack by the incoming
nucleophile

H Attack is very easy!
H C Br
Attack is easy!
H
Attack is slightly
methyl bromide difficult!

H

HC Br
CH3

ethyl
bromide

(1o)

H

CH3 C Br
CH3

isopropyl bromide
(2o)

ORDER OF REACTIVITY

Increasing rate of an SN2 reaction

R3CX < R2CHX < RCH2X <
C3Ho3X
2o 1o methyl

more crowded less crowded
more steric hindrance less steric hindrance
less reactive more reactive

EXERCISE

Give the SN2 mechanism for the formation
of the product in the following reaction.

CH3CHBrCH2CH3 + OH– CH3CHOHCH2CH3 + Br –

SN1
MECHANISM

❶ Unimolecular Nucleophilic Substitution

First step involve only one molecule (haloalkane)

The rate of SN1 reaction does not depend
on the concentration of nucleophile.

The rate depends only on the
concentration of the substrate, alkyl
halide.

Rate = k [R-X]

* SN1 is a first order reaction

SN1 MECHANISM

❷ First–order reaction

(CH3)3C—Cl + OH– (CH3)3C–OH + Cl–

Rate = k[(CH3)3CCl]
• depend on [(CH3)3CCl]
• first order overall

Only (CH3)3CCl involved in
the step that control reaction rate
☞ unimolecular

SN1 MECHANISM

❸ Two–step mechanism
❑ Carbocation is formed as a reactive
intermediate

STEP 1 Formation of Carbocation

3o haloalkane 3o carbocation

Step 1 (slow) the rate–determining
step (slowest step)

SN1 MECHANISM

❸ Two–step mechanism

STEP 2 Nucleophilic attack on the Carbocation

Write the mechanism for the following
reaction

+ H2O

Solution + HBr
> 3o haloalkane
> weak nucleophile
> 2 steps mechanism involve, SN1

SN1 MECHANISM

Step 1 : Formation of carbocation

CH3 slow H3C CH3

H 3C C Br -

CH3 C + Br

CH3

3o Carbocation

SN1 MECHANISM

Step 2: Nucleophilic attack on the carbocation

H3C fast H3C H
H3C C
+ O..+ H
CH3
H3C C

CH3

H3C H H3C + H3O+
H3C C OH
H3C C O..+ H
CH3
CH3

Write the mechanism for the following reaction.

Solution
10 haloalkane but more steric effect

Has rearrangement

44

SN1 MECHANISM

Step 1 : Formation of carbocation

CH CH3 _B....r slow CH3 .. _
_C _CH CH 3_ C _C+H 2 B..r
3 2 ..+
..

..

CH 3 CH3

Rearrangement :

CH3 1,2-methyl shift _ CH 3
CH 3 _ C _C+H 2 _C
C H 3 C H 2
CH 3 +
CH3

1o carbocation 3o carbocation

SN1 MECHANISM

Step 2 : Nucleophilic attack on the carbocation

_ CH 3 CH 3
_ fast CH 3_C _CH 2CH 3
C H 3 C C H 2
+ +O H
CH3 H
..
..CH 3
CH 3_ C _CH 2CH 3 CH 3
+
H CH 3 _ C _CH 2 CH 3 + H3O+
O
OH

H

EXERCISE

Give the SN1 mechanism for the formation
of the product in the following reaction.

CH3 CH3
CH3—CH—CH—CH3 CH3OH CH3—C—CH2—CH3

Br OCH3

ORDER OF
REACTIVITY

Reactivity and reaction rate determined by the
stability of carbocation formed

Increasing rate of an SN1 reaction

CH3X < RCH2X < R2CHX < 3o
Rm3eCthXyl 1o 2o

form least form most
stable carbocation stable carbocation

TYPES OF NUCLEOPHILIC
ALKYL HALIDES SUBSTITUTION

1o and CH3X MECHANISM
2o
SN2
3o
SN2 or SN1

SN1

2o alkyl halide undergo both SN1
and SN2 reactions. Other factors
determine the mechanism

EFFECT OF NUCLEOPHILE

SN1 ❑ Nucleophile strength is
unimportant

• Favored by weak nucleophiles
(usually neutral)

SN2 ❑ Strong nucleophiles is required
• Favored by strong nucleophiles
(usually a net negative charge)

SN1 or SN2 ???

Haloalkanes If low steric effect SN2
If high steric effect SN1
1o SN2

SN1 : if weak nucleophile used
2o e.g: RNH2 , ROH

SN2 : if strong nucleophile used
e.g: OH-, CN-, I-, RO-

3o SN1

SN1 MECHANISM SN2 MECHANISM

Unimolecular Nucleophilic Bimolecular Nucleophilic
Substitution Substitution
- First step involve only one molecule - The reaction involve two molecule
(Haloalkane) (Haloalkane & Nucleophile)

First Order reaction Rate = k [RX] Second Order Reaction
Rate = k [RX][Nu-]

Carbocation as intermediate No carbocation, Has a transition
Two steps mechanism state

One step mechanism

Order of reactivity : Order of reactivity :
Methyl halide < 1° < 2° < 3° 3°< 2° < 1° < Methyl halide

Weak nucleophiles (usually neutral Strong nucleophile (usually -ve charge
species) ion)

ELIMINATION REACTION OF
HALOALKANES

❑ Haloalkanes undergo elimination
reactions with bases to form alkene.

❏ Name of reaction:
Dehydrohalogenation of haloalkanes

CH3 KOH, Ethanol CH3
CH3–C–CH3
CH3–C═CH2 + HCl
Cl
reflux

refer subtopic 5.2

i. Br

CH3CHCHCH3 CH 3CH 2ON a CH3C CHCH3
CH 3CH 2OH

CH3 reflux CH3

major

+

CH 3CH CH CH 2

CH3

minor

ii.

reflux major minor 54

USE OF HALOALKANES IN THE
SYNTHESIS

OF GRIGNARD REAGENTS

❑ Synthesis of Grignard Reagents (R- Mg-X)
by the reaction of haloalkanes with
magnesium metal in anhydrous (dry) ether
as a solvent.

Grignard Reagent
( alkylmagnesium halide)

55

CH3Br + Mg ether CH3MgBr
methyl bromide

CH3CH2Br + Mg ether CH3CH2MgBr
ethyl bromide

CH3CH2CHCl + Mg ether CH3CH2CHMgCl
CH3
CH3
sec–butyl chloride

GRIGNARD REAGENTS ( RMgX )

Haloalkanes (R-X) + Mg

dry ether
Grignard Reagents

(RMgX)

Uses of RMgX

H2O/H+ [1]RCOR’, ether [1]CO2
ALKANES [2]H3O+ [2]H3O+
R’ = H or alkyl
ALCOHOLS CARBOXYLIC
ACIDS

Uses of Grignard Reagent

O
[1] H -C - H , ether
R-H H3O+ (methanal) H - OH
(alkane) C -H

[2] H3O+ (1o aRlcohol)

O
[1] H -C - R , ether OH
R-MgX (aldehyde) H- C- R

Grignard [2] H3O+ (2o R
alcohol)

reagent O
R-C-R ,
OH CO2 [1] (ketone) ether OH
R- C=O C- R
(carboxylic acid) [1] H3O+ R -
[2]
[2] H3O+ (3o aRlcohol)

58

Uses of Grignard Reagent

SyntheRsi-sMofgAXlkane

i. CH3CH2–Br + Mg dry ether CH3CH2–MgBr 59

H3O+

CH3CH3
MgCl
ii. CH3CHCH3 + Mg dry ether CH3CHCH3
Cl H3O+

CH3CH2CH3

Uses of Grignard Reagent

SynthesiRs o-Mf 1goXAlcohol

Cl MgCl + O
CH3CHCH3 + Mg dry ether CH3 CHCH3 H-C -H

H3O+

OH
H-C -H

CH3 CHCH3

60

Uses of Grignard Reagent

SynthesiRs o-Mf 2goXAlcohol

CH3–Br + Mg dry ether O
CH3 MgBr + H- C -CH3

H3O+

OH
H -C - CH3

CH3

61

Uses of Grignard Reagent

SynthesiRs o-Mf 3goXAlcohol

CH3CH2–Cl + Mg dry ether O
CH3 CH2 MgCl + CH3-C-CH3

H3O+

OH

CH3-C-CH3
CH3 CH2

Uses of Grignard Reagent

Synthesis Rof-CMagrbXoxylic Acid

CH3CH2–Cl + Mg dry CH3 CH2-MgCl + CO2
ether H3O+

O
CH3 CH2- C-OH

63

END OF SLIDE SHOW

Chapter 8

HYDROXY
COMPOUNDS

8.2 Physical Properties

8.1 Introduction 8.3 Preparation

8.0 HYDROXY 8.5 Phenol
COMPOUNDS

8.4 Chemical Properties

8.1 LEARNING OUTCOMES

Introduction

a) Give the name of hydroxy compounds according
to the IUPAC nomenclature.
b) Give the structural formulae for the hydroxy
compounds (parent chain ≤ C10).
c) classify the hydroxy compounds

INTRODUCTION TO ALCOHOLS

❑ Contain hydroxyl group (-OH) bonded to

sp3 hybridized C atom.

❑ Aliphatic alcohol
General formula : R—OH
☞ CnH2n+1OH

CLASSIFICATION OF ALCOHOLS

❑ Depending on type of C atom to which the
–OH group is directly attached.

CLASS GENERAL FORMULA EXAMPLE
1o
2o R–OH CH3CH2OH
R–CHR
3o CH3CHCH3
OH OH
R CH3

R–C–R CH3C–CH3

OH OH

EXERCISE 21 :

Classify the types of alcohol (1o, 2o or 3o)
for the following molecules.

(a) CH3CH(OH)CH3 (b) (CH3)2C(OH)CH2CH3

CH3
(c) (d)

CH2OH OH

H3C CH3

NOMENCLATURE OF ALCOHOLS

1) Find the longest C chain containing the OH.
2) Give the –OH group the lowest number.
3) Identify substituent groups and their position.
4) The substituent are arranged in alphabetical order.
5) The suffix ‘–e’ in the alkane parent name is replaced

by ‘–ol’.

Example:

OH

1 23 4

CH3 C CH2CH3

CH3

2–methyl–2–butanol

CH3 OH
CH3CHCH2CHCH2CH3

6 54 32 1

5–methyl–3–hexanol

6) Ring/Cyclic: Numbering start at C bearing OH.

EXAMPLE:

5 6 1 OH

4 2
3
Cl

2–chlorocyclohexanol

CH3

5
4

1

OH

32

Br

2–bromo–5–methylcyclopentanol

Example:

Structure Common name IUPAC name
methanol
CH3OH methyl alcohol ethanol
2-propanol
CH3CH2OH ethyl alcohol

CH3CH(OH)CH3 isopropyl alcohol

OH cyclohexyl cyclohexanol

alcohol

10

EXAMPLE:
CH3CH2CH2CH2OH
• 1979 IUPAC recommendation: 1–butanol
• 1993 IUPAC recommendation: butan–1–ol

The first of these convention
☞ 1–butanol is more widely used

7) Aromatic alcohol – phenol

(-OH group attached directly to benzene ring)

For phenol, C attach to the –OH group is C1.

Example:

OH CH3 NO2
phenol
OH OH
3–nitrophenol
2-methylphenol
or or m–
nitrophenol
o-methylphenol

8) –OH has priority over C═C
EXAMPLE:

12 34 5

HO CH2 CH CH CH2 CH3

2–penten–1–ol or pent–2–en–1–ol

OH

1 2 2–cyclohexen–1–ol
6
or
53 cyclohex–2–en–1–ol
4

Increasing PRIORITY MAIN GROUPS
fo parent name carboxylic acids
esters
aldehydes
ketones
alcohol
amine
alkenes
alkanes / halides