SUMMARY OF CHAPTER 7 HALOALKANES
- General formula: R—X ; X = F, Cl, Br, I
- Also called Alkyl Halides
- Classification: Depends on the classification of C that bonds to the halogen
- IUPAC nomenclature for Haloalkanes follows the basic rules as described in alkanes
- Characteristics: POLAR C—X BOND (The electronegative halogen X creates a polar C–X bond, making C atom
electron deficient; the carbon is electrophilic and the halogen is nucleophilic; The electrophilic C of haloalkanes is
susceptible to nucleophilic attack)
- Haloalkanes undergo elimination (example dehydrohalogenation: RX in alcoholic KOH and heated produced
alkene + HX) and substitution reaction (example: R-X + :Nu- → R-Nu + :X-)
Strong Weak *Sometimes CH3COO- can
nucleophile nucleophile consider as intermediate
OH- H2 O nucleophile*
- -
OR ; e.g: CH3O ROH ; e.g: CH3OH
CN - NH3
- Two mechanisms: SN2 (Bimolecular Nucleophilic Substitution) and SN1 (Unimolecular Nucleophilic Substitution)
CHEMICAL REACTIONS:
1. Reaction with NaOH: R-X + OH- → R-OH + X-
2. Reaction with alkoxide ion: R-X + -OR (alkoxide) → R-OR (ether) + X—
3. Reaction with excess ammonia: R-X + NH3 (excess) → R-NH2 (1o amine) + NH4+X-
4. Reaction with KCN or NaCN: R-X + -C≡N (cyanide) → R-C≡N (nitrile) + X-
5. Reaction with alcohol: R-X + ROH → R-OR (ether) + HX
6. Reaction with acetate: R-X + CH3COO- (acetate) → R-COOCH3 (ester) + X-
7. Reaction with H2O: R-X + H2O → R-OH + HX
REACTIVITY OF HALOALKANES
The bond dissociation energies of C-X bonds follow the order : C−F > C−Cl > C−Br > C−I. Since the more the bond
dissociation energy, the more difficult to break the bond. The reactivity follows the order : RI > RBr > RCl > RF .
Types of alkyl halides Nucleophilic substitution mechanism SN1 or SN2 ? Factors are examined;
1o , CH3X SN2 1. Types and reactivity of alkyl halides:
SN2 or SN1 (the mechanism depends on the methyl halides, 1⁰, 2⁰ or 3⁰
2o 2. Strength of nucleophile: strong or weak
strength of nucleophile)
3. Other informations from the question: rate
of reaction, products formed (example
3o SN1 major and minor products mean SN1
prefer) or etc
COMPARISON SN1 MECHANISM AND SN2 MECHANISM
SN1 MECHANISM SN 2 MECHANISM
Name Unimolecular Nucleophilic Substitution Bimolecular Nucleophilic Substitution
- First step involve only one molecule :Haloalkane - The reaction involve two molecule:
(the rate determining step) Haloalkane & Nucleophile ((in the rate
determining step)
Kinetics (Rate) First Order reaction: Rate = k [RX] Second Order Reaction:
(Rate of reaction is proportional to the concentration Rate = k [RX][Nu-]
of haloalkane only) (Rate of reaction is proportional to the
concentration of both haloalkane and
nucleophile)
Intermediate Carbocation as intermediate Transition state as intermediate
Rearrangement May occur to form a stable carbocation No carbocation rearrangement//
Rearrangement is impossible
SN1 MECHANISM SN 2 MECHANISM
Effect of Nucleophile strength are unimportant Strong nucleophile are required
Nucleophile (usually a species with negative charge)
Step of Two steps mechanism: One step mechanism
mechanism 1. The first step is a slow ionisation to form a (Has a transition state and no
carbocation (as an intermediate): rate determining carbocation rearrangement)
step
2. The second step is a fast attack on the
carbocation by a nucleophile
Type of Weak nucleophile Strong nucleophile
nucleophile
Reactivity of Order of reactivity : Order of reactivity :
RX R3CX > R2CHX > RCH2X > CH3X CH3X > RCH2X > R2CHX> R3CX
3⁰ 2⁰ 1o methyl halide Methyl halide 1⁰ 2⁰ 3o
(3o> 2o; 1⁰ and CH3X are unlikely) (CH3X > 1o > 2o ; 3⁰ RX is unsuitable:
(Forming most stable carbocation, more susceptible Steric hindrance caused by bulky R groups
for nucleophile attack) makes nucleophilic attack from the
backside more difficult and they slow the
reaction rate )
Mechanism : Mechanism :
Step 1 : Formation of carbocation
Step 2 : Reaction of carbocation with nucleophile
(A) Example A
Mechanism:
Example:
Reaction: HO– + CH3Cl → CH3OH
+ Cl–
(B) Example B
*If the nucleophile is water or an alcohol, a third step
is needed to deprotonated the product* - All SN2 reactions proceed with
Mechanism: backside attack, resulting in inversion
Step 1 of configuration at the electrophilic C
atom
- The nucleophile approaches for the
side opposite (180o) the leaving group
Step 2
Step 3
GRIGNARD REAGENTS ( RMgX )
- Reactants: alkyl / aryl halide (R-X) and Mg
- Condition: Solvent: dry ether
Dry ether
R-X + Mg R-Mg-X
Info:
1. Aryl halide: is an
aromatic compound in
which one or more
hydrogen atoms directly
bonded to an aromatic ring
are replaced by a halide.
2. Alkyl halide also
known as haloalkane
USES OF RMgX → In the synthesis of:
(a) ALKANE
(b) ALCOHOLS
(c) CARBOXYLIC ACIDS
