Halogenoalkanes, also known as alkyl halides, contain carbon-halogen bonds. They can be synthesized through free radical substitution or electrophilic addition reactions. Nucleophilic substitution reactions of halogenoalkanes produce alcohols or other products depending on the solvent. In aqueous solutions, hydroxide acts as a nucleophile to form alcohols via SN1 or SN2 mechanisms. In alcoholic solutions, hydroxide acts as a base to eliminate halogens and form alkenes. Both substitution and elimination reactions occur simultaneously but the solvent influences which pathway dominates.

1. 8. Alkyl Halides
Dr Wong Yau Hsiung
CHEM 221

2. McMurry Organic Chemistry 6th edition, Chapter 10
(c) 2003
2
What Is an Alkyl Halide/
Haloalkanes
 An organic compound containing at least one carbon-
halogen bond (C-X)
 X (F, Cl, Br, I) replaces H
 Can contain many C-X bonds
 Properties and some uses
 Fire-resistant solvents
 Refrigerants
 Pharmaceuticals and precursors

3. • Halogenoalkanes are similar
to alkanes but with one or
more of the hydrogen atoms
replaced by a halogen.
• Halogenoalkanes can
contain more than one type
of halogen. For example, CFCs
(chlorofluorocarbons) contain
both chlorine and fluorine
atoms. chloro-pentafluoroethane
trichloromethane
What are Haloalkanes?

4. • Name is based on longest carbon chain (contains double or triple
bond if present)
• Number the carbons of the parent chain beginning at the end
nearer the first substituent, whether alkyl or halo
• If more than one of the same kind of halogen is present, use
prefix di, tri, tetra
• If there are several different halogens, number them and list
them in alphabetical order
Naming Haloalkane

5. • Naming if two halides or alkyl are equally distant from ends of
chain
- Begin at the end nearer the substituent whose name comes first in
the alphabet
Naming Haloalkane

7. A chain of carbon atoms can be represented by R when drawing the
structure. This is referred to as an R group.
 Primary (1°) halogenoalkanes
have one R group attached to
the carbon linked to the halogen.
 Secondary (2°) halogenoalkanes
have two R groups attached to the
carbon linked to the halogen.
 Tertiary (3°) halogenoalkanes
have three R groups attached to
the carbon linked to the halogen.
Primary, secondary and tertiary
R-CH2-X
R2-CH-X
R3-C-X

9. Physical Properties
Solubility : All organic halides are insoluble in water and soluble
in common organic solvents.
Boiling point : The boiling points increases with increasing in
molecular weights.
Therefore, the boiling points increases in the order F<Cl<Br<I.
BP also increases for “straight” chain isomers.
Greater branching = lower BP

10. How are halogenoalkanes made?

11. There are several ways by which halogenoalkanes can be
made, including:
 free radical substitution of an alkane:
 electrophilic addition of HX or X2 to an alkene:
CH4 + Cl2 → CH3Cl + HCl
C2H4 + HBr → C2H5Br
C2H4 + Br2 → C2H4Br2
How are halogenoalkanes made?

13. • The carbon–halogen bond in halogenoalkanes is polar because
all halogens are more electronegative than carbon.
• The polar bond means that the carbon atom has a small
positive charge (δ+), which attracts substances with a lone pair
of electrons. These are nucleophiles, meaning ‘nucleus
(positive charge) loving’. Examples include:
δ+ δ- δ+ δ- δ+ δ- δ+ δ-
ammonia cyanide hydroxide
Polar bonds and nucleophiles

14. • The nucleophile uses its lone pair to provide the electrons for a
new bond
• The halogen is displaced
• The result is substitution following attack by a nucleophile
Nucleophilic Substitution
• The general form for the reaction is
Nu:-
+ R-X  R-Nu + X:
Nucleophile Substrate Product Leaving group

15. Reagent Aqueous sodium (or potassium) hydroxide
Conditions Reflux in aqueous solution (SOLVENT IS IMPORTANT)
Elimination takes place when ethanol is the solvent
The reaction with water is known as HYDROLYSIS
Product Alcohol
Nucleophile hydroxide ion (OH¯)
Equation C2H5Br(l) + NaOH(aq) —> C2H5OH(l) + NaBr(aq)
Nucleophilic Substitution – Aqueous NaOH

18. Nucleophilic Substitution Bimolecular or SN2
• The rate depends on the conc. of 2 reactants: the substrate and
the nucleophile.
• Takes place in one step
• No carbocation
• new bond forming and old bond breaking at same time.
• Occur most readily with methyl compounds and primary
haloalkanes
H3CH2C
C
CH3
H
BrOH-
H3CH2C
C
CH3H
BrOH
CH2CH3
C
CH3
H
OH
Transition State:
As OH- attaches,
Br- leaves
+ Br-+

19. Nucleophilic Substitution Bimolecular or SN2
• Transition state is highest in energy.

20. Nucleophilic Substitution Unimolecular or SN1
• The rate depends on the conc. of 1 reactant: the substrate but not
the nucleophile.
• A two step process since the substrate and the nucleophile cannot
both appear in the rate determining step
• Form carbocation intermediate
• Occur most readily with tertiary haloalkanes and some secondary
haloalkanes.
H3CH2C
C
CH3
H
Br
H3CH2C
C
+
CH3H
CH2CH3
C
CH3
H
OH
Transition State:
Formation of
Carbocation
Br-
OH-
+
Rate
determining
step: spotaneous
dissociation of
leaving group
Very fast step:
reaction of
nucelophile and
carbocation

21. Nucleophilic Substitution Unimolecular or SN1

22. SN1 and SN2
SN1 SN2
Rate =k[RX] =k[RX][Nuc:-
]
Carbocation
intermediate?
Yes No
Number of steps 2 1
Occurs with Tertiary
halogenoalkanes
Primary
halogenoalkanes

23. • In the reaction with a strong base, halogenoalkanes will undergo
not only nucleophilic substitution but also elimination reactions,
forming alkenes and water.
• The OH-
acts as both a base and a nucleophile. When acting as
a base, the OH-
removes H+
from the halogenoalkane, which
also results in the formation of a halide ion.
• The reaction between a halogenoalkane and a strong base
usually results in the formation of a mixture of substitution
and elimination products.
Elimination in Haloalkanes

24. Reagent Alcoholic sodium (or potassium) hydroxide
Conditions Reflux in alcoholic solution
Product Alkene
Mechanism Elimination
Equation C3H7Br + NaOH(alc) —> C3H6 + H2O + NaBr
Complication With unsymmetrical halogenoalkanes, you can get
mixture of products
Elimination

25. • If the carbon chain is four or more carbons in length and the halogen
is not attached to a terminal carbon, a mixture of positional isomers
may be formed.
attack at A
attack at BA B
but-2-ene but-1-ene
Mixture of elimination products
can exist as cis and trans isomers

26. Zaitsev’s Rule for Elimination Reactions
• In the elimination of HX from an alkyl halide, the more highly
substituted alkene product predominates
• If more than one elimination product is possible, the most-
substituted alkene is the major product (most stable).
• R2C=CR2 > R2C=CHR > RHC=CHR > H2C=CHR
tetra > tri > di > mono

28. The products of reactions between haloalkanes and OH¯ are influenced by the solvent
SOLVENT ROLE OF OH– MECHANISM PRODUCT
WATER NUCLEOPHILE SUBSTITUTION ALCOHOL
ALCOHOL BASE ELIMINATION ALKENE
Modes of attack
Aqueous solution
•OH¯ attacks the slightly positive carbon bonded to the halogen.
•OH¯ acts as a nucleophile
Alcoholic solution
•OH¯ attacks one of the hydrogen atoms on a carbon atom adjacent the carbon
bonded to the halogen.
•OH¯ acts as a base (PROTON ACCEPTOR)
Both reactions take place at the same time but by varying the solvent you can
influence which mechanism dominates.
Elimination vs Substitution