CHAPTER 4
ALKYL HALIDE
BY
MAHWASH HAFEEZ
Alkyl Halides
Alkyl halides have the general formula R-X
R is alkyl group (functional group) and X is the
halogen atom ( CL- Br-F)
Alkyl groups are alkanes with one hydrogen removed.
It is named by replacing the “-ane” of the alkane with
“–yl”.
Alkyl group + halides = alkyl halide.
Categories of alkyl halides
R-X
(X = F, Cl, Br, I)
According to the class of the carbon that the halogen is attached to,
alkyl halides are classified into three categories.
RCH2-X
1o
Primary
R2CH-X
2o
secondary
R3C-X
3o
tertiary
Primary
secondary
tertiary
1- Nomenclature (common name)
• Replace ( ane ) from alkane and addition ( yl )
and add name of halogen ( X )
Methane
Methyl
ethane
ethyl
propane
propyl
Hexane
Hexyl
Halide( halogen
)
Cl
I
Br
F
chloride
Iodide
Bromide
Flouried
 Methyl Chloride
 Ethyl chloride
 propyl bromide
 Butyl iodide
 Pentyl chloride
→
→
→
→
→
CH3-CL
CH3-CH2-CL
CH3CH2CH2-Br
CH3CH2CH2CH2 I
CH3CH2CH2CH2CH2-CL
2- Nomenclature for Alkyl Halides)IUPAC( :
1.
2.
The longest continuous chain of carbon atoms containing the halogen atom is
the parent.
Number the parent chain from the end nearest the halogen atom.
CH3
‫׀‬
CH3 – CH – CH2 – Br
1-bromo-2-methylpropane
CH3I
CH2I2
iodomethane
diiodomethane
methyl iodide
methylene iodide
CHI3
CI4
triiodomethane
tetraiodomethane
iodoform
carbon tetraiodide
Physical Properties
• Alkyl halides are weak polar molecules. They exhibit dipole-dipole
interactions because of their polar C—X bond.
• Alkyl halide have higher boiling points than alkane with the same
number of carbon
• Alkyl halides have relatively low melting points
• Alkyl halides are insoluble in water because of their in ability to
form hydrogen bonding.
8
SYNTHESIS OF ALKYL HALIDE :
1-from alcohols:
a) With hydrogen halide (HX):
CH3CH2-OH
+ HI
 CH3CH2-I + H2O
ethyl alcohol
ethyl iodide
ethanol
iodoethane
Alcohol tend to under go rearraangement during replacement of OH by X
b) With phosphorus halide (PX3):
CH3
|
CH3CHCH2-OH
isobutyl alcohol
2-methyl-1-propanol
+
PBr3
CH3
|
 CH3CHCH2-Br
isobutyl bromide
1-bromo-2-methylpropane
2. Halogenation of certain hydrocarbons:
Alkanes react with halogens upon heating to form
alkyl halides.
R-H
+ X-X
Δ or hν
R-X + HX
(requires Δ or hν; Cl2 > Br2 (I2 NR); 3o>2o>1o)
CH3
CH3
‫׀‬
CH3 – C – CH3
‫׀‬
CH3
2,2-dimethylpropane
‫׀‬
+ Cl2
heat
CH3 – C – CH2 – Cl
‫׀‬
CH3
1-chloro-2,2-dimethylpropane
3- Chlorination of methane
CH4 +
Cl2
hv
CH3Cl + HCl
methyl chloride
(chloromethane)
CH3Cl
+ Cl2
hv
CH2Cl2 + HCl
methylene chloride
(dichloromethane)
CH2Cl2 + Cl2
hv
CHCl3 + HCl
chloroform
(trichloromethane)
CHCl3
+
Cl2
hv
CCl4 + HCl
carbon tetrachloride
(tetrachloromethane)
4- Addition of hydrogen halides to alkene:
CH2=CH2+ HCl → CH3-CH2-Cl
Ethene
Ethyl chloride
5- Addition of halides to alkane:
Alkyl Halides and Nucleophilic
Substitution
• Alkyl halides are organic molecules containing a halogen
atom bonded to an sp3 hybridized carbon atom.
• Alkyl halides are classified as primary (1°), secondary
(2°), or tertiary (3°), depending on the number of carbons
bonded to the carbon with the halogen atom.
• The halogen atom in halides is often denoted by the
symbol “X”.
14
Nucleophilic Substitution
A nucleophile can donate a pair of electrons to the
electrophilic carbon atom of the alkyl halide forming
a new bond and displacing the halide.
SN1 reaction
• It involves two steps:
*Slow step (rate determining):
Ionization of the alkyl halide to form a
carbocation(intermediate).
*Fast step:
Addition of the nucleophile to the
carbocation.
Rate = k[RX]
First order reaction
MECHANISM:
Bond breaking occurs before bond making.
17
EXAMPLE:
Mechanism:
This first step involves an initial ionisation of the t- butyl bromide
(2-bromo–2–methyl propane) forming a carbocation:
Bond breaking
step
Slow step
2-bromo–2–methyl propane
Carbocation
The second step involves a very rapid attack by the hydroxide ion
on the carbocation (carbonium ion) to form 2–methyl–2– Propanol:
+ Na+
Carbocation
Bond forming
step
Fast step
2–methyl–2– Propanol
19
SN2 Mechanism
• An SN2 reaction takes place in one step - attack of
the nucleophile on the alkyl halide. This reaction
• does not have an intermediate. A transition state is
formed in which the bond between the nuclophile
• and carbon atom is being formed at the same time
the bond between the carbon atom and the leaving
• group is being broken.
Rate = k[RX][Z-]
Rate = k[RX][Z-]
Second order reaction
Reaction Of alkyl Halides
1. Dehydrohalogenation of alkyl halides:
When heated with strong bases, alkyl halides typically undergo a 1,2elimination reactions to generate alkenes.
2. Reaction with Metals:
Alkyl halides react with metals to give Grignard reagent.
CH3 – Cl
Mg
ether
CH3 – Mg –Cl
3. Willaimson ether synthesis:
An ether is prepared by the nucleophilic substitution (typically SN2 reaction )
of an alkyl halide with an alkoxide ion.
C2H5OH
Ethyl alcohol
Na
C2H5
O–Na+
Alkoxide
C2H5Cl
NaCl
C2H5 – O – C2H5
Di-ethyl ether
4. Hydrolysis:
On hydrolysis alkyl halides yield alcohols.
H – OH
CH3 – Br + NaOH
CH3 – OH + NaBr
5. Kolbe nitrile synthesis:
Alkyl halides undergo nucleophilic substitution with alkali
metal cyanides to form alkyl nitriles.
Uses of alkyl halides
1. Chloroflourocarbons are alkyl halides.e.g. Feron – 12. It was used as a
coolant in refrigerators. However its use is now banned due to the damage
this compound has caused to ozone layer.
2. 1,1,1-trichloroethane ( an alkyl halide) is used in making degreasing agents,
glue, paint, and cleaning products.
3. Carbon tetrachloride is also an alkyl halide which is used in fire extinguishers,
dry-cleaning businesses, and cleaning products.
4. Bromomethane is implemented in many pesticides as well as fumigants. As a
fumigant it is used to help contain the spreading and overgrowth of certain
fungi, nematodes, and weeds.
5. Chloroform is a clear liquid with an ether-like odor and a slightly sweet taste. It
is used as anesthetic, to make coolants and as a dry cleaning spot remover.