CI 13.1 Halogenoalkanes
Are man-made compounds with one or more halogen atoms (F,
Cl, Br, I) attached to a carbon atom. The attached halogen changes
the chemical properties of alkane chains…they are very
unreactive, and so have been very useful to humans.
Naming halogenoalkanes (haloalkanes)
(similar rules to naming alcohols, just add the halogen as a prefix):
 halogens are in alphabetical order.
 lowest numbers possible are used.
CH3CH2CH2Cl
is
1-chloropropane
CH3CHClCH2Cl
is
1,2-dichloropropane
CH3CHBrCH2CH2Cl
is 3-bromo, 1-chlorobutane.
CH3CHICHBrCH2Cl
2-bromo,1-chloro,3-iodobutane
2-bromo, 3-chloro, 1-iodopentane
CH2ICHBrCHClCH2CH3
Physical properties of halogenoalkanes
 immiscible with water
 The bigger the halogen atom /the larger the number of
halogen atoms the higher the boiling point.
 Larger halogen atoms (Br or Cl) cause greater environmental
damage than smaller halogen atoms (F); this is important
when designing replacements for CFCs.
Chemical reactions of halogenoalkanes
Carbon –halogen (C-Hal) bonds can break either homolytically or
heterolytically.
Homolytic Fission forms radicals eg when a halogenoalkane
absorbs radiation of the right frequency.
H
H
H
C
Cl
+
hv

H
H
C
+
Cl
H
Chloromethane
methyl radical
Shorthand is: CH3-Cl + hv  CH3 + Cl
chlorine radical
(occurs in stratosphere).
Heterolytic fission is more common in lab conditions using polar solvents
such as ethanol or ethanol and water. The polar C-Hal bond can break,
leaving a negative halide ion and positive carbocation.
CH3
CH3
C
CH3
Cl
+
hv

H
CH3
C+ + ClH
2-chloro-2-methylpropane
carbocation
chloride ion
(negatively charged substances may react with the positive carbocation
causing a substitution reaction).
Importance of reaction conditions…for determining how bonds break
Eg. Bromoethane C-Br bonds break:
 Heterolytically, forming ions when dissolved in a polar solvent
(say a mixture of ethanol and water) BUT
 Homolytically, in the gas phase at high temp. or when dissolved in a
non-polar solvent, such as hexane.

Different halogens, different reactivity.
All reactions with halogenoalkanes involve breaking the C-Hal bond. The
C-F bond is the strongest (bond enthalpy 467 kJmol-1) and therefore the
hardest to break, whereas the C-I bond is relatively weaker (228 kJmol-1)
and therefore easier to break. C-Hal bonds get weaker, and so more
reactive, down group 7.
Chloro compounds are fairly unreactive and remain in the troposphere
long enough to reach the stratosphere, where they react with and destroy
the ozone layer.
Substitution reactions of halogenoalkanes
Halogenoalkanes can hydrolysed by hydroxide ions to form alcohols.
Eg. Bromobutane forms butanol:
CH3–CH2–CH2–CH2–Br + OH-  CH3–CH2–CH2–CH2–OH + BrThe C-Br bond is polar



C–Br
The oxygen atom on OH- is –vely charged.




H–O
The partial positive charge on the carbon atom attracts the negatively
charges oxygen of the hydroxide ion. A lone pair of electrons on the O
atom forms a bond with the C atom as the C__Br bond breaks.
H H H H
H C__C__C__C__Br
H H H H
H H H H
H C__C__C__C__O__H
H H H H
__
__
_
O
H
_
+
Br
Heterolytic fission results in IONS and not radicals.
Curly arrows show the movement of electrons (full headed
arrows for a pair of electrons…unlike radical reactions).
Halogenoalkanes can give substitution reactions with hydroxide ions
and other NUCLEOPHILES. Nucleophiles can donate a pair of electrons
to a positively charged carbon atom to create new covalent bonds.
Some common nucleophiles:
Name
Formula
Hydroxide ion
OH-
Structure showing
lone pairs
_
H__O
CH3COO-
Ethanoate ion
_
__
__
CH3 C O
O
C2H5O-
Ethoxide ion
_
__
CH3CH2 O
Water molecule
H2O
O
H
Ammonia molecule
H
NH3
N
H
H
H
CN-
Cyanide ion
_
N
C
The carbon atom attacked by the nucleophile may be part of a
carbocation and carry a full positive charge, or it may be part of a
neutral molecule (as in the above example with bromobutane) and carry a
partial positive charge as a result of bond polarisation.
If X- represents a nucleophile, the nucleophilic substitution process is:
X
 
C–Hal 





__
C X + Hal







Water as a nucleophile
Nucleophiles may be neutral or have negative charge, so long as it has a
lone pair of electrons which can form a bond to a carbon atom.
Eg. Water has 2 lone pairs of electrons on the O atom. First it attacks the
halogenoalkane (bromobutane in this case):
H H H H
H C__C__C__C__Br
H H H H
H H H H
H C__C__C__C__O__H
H H H H H
__
__
_
+
Br
+
H+
O
H
H
The resulting ion loses H+ to form an alcohol:
H H H H
+
__ __ __ __
H C C C C O
H H H H H
__
H H H H
H C__C__C__C__O__H
H H H H
__
The overall equation for the reaction of water with a genera;
halogenoalkane R__Hal is:
R__Hal + H2O  + R__OH + H+ + Hal-
Ammonia as a nucleophile
A lone pair of electrons on the N (similar to water) attacks the
halogenoalkane to produce an AMINE with an NH2 group:
R__Hal + NH3  R__NH3+ Hal-
R__NH2 + H+ + Hal-
Using nucleophilic substitution to make halogenoalkanes
Halogenoalkane + OH-
alcohol
Halogenoalkanes can be made via the reverse reaction of making alcohols;
the nucleophile is Hal-.
Eg. 1-bromobutane is made using a nucleophilic substitution reaction
between butan-1-ol and Br- ions, in the presence of a strong acid.
Ist step: H+ ions bond to O atom on the alcohol:
H H H H
H H H H
+
__ __ __ __
__ __ __ __ __ __
H C C C C O
H C C C C O H
H H H H
H H H H H
H+
This gives the C atom to which the O is attached a greater partial positive
charge. It is now more readily attacked by Br- ions, forming bromobutane.
__
H H H H
+
H__C__C__C__C__O
H H H H H
_
Br
H H H H
H__C__C__C__C__Br
H H H H
+
H 2O
The overall equation for the reaction is:
CH3CH2CH2CH2OH + H+ + Br-  CH3CH2CH2CH2Br + H2O
 Activity A4.2
 Problems for 13.1 pages 303- 304
questions 1- 9.