Alkyl Halides
By cleavage of one hydrogen atom
from an alkane, an alkyl group forms. Table shows some alkyl groups derived
from alkanes.
When a halogen atom is bonded to
an alkyl group, the resulting com- pound is called as an alkyl halide.
Alkyl halides are divided into
three according to the carbon atom that halogen is attached to: primary (1°),
secondary (2°) and tertiary (3°)
While forming alkyl halides,
halogen atoms can replace more than one hydrogen atoms either on the same
carbon or different ones.
The Nomenclature of Alkyl Halides
Alkyl halides are named according
to IUPAC nomenclature system as follows:
1. The longest carbon chain which has halogen atom is
chosen. It is num- bered starting from the halogen atom.
2. Firstly, the number of the carbon atom attached to
halogen atom is written. Then, following the (-) hyphen symbol, halogen’s name
is writ- ten with –o suffix added (chloro, bromo, iodo). If there are other
halo- gens, first the (-) symbol, then the names of those halogens are written
in the same way. Halogens’ names are written in alphabetical order in the
compound. For example, bromo, chloro, iodo respectively. You can see those
orders in Table 2.
The Synthesis of Alkyl Halides
Alkyl halides are synthesized
through many methods in industry and laborato-
ries. We will explain some of
those laboratory methods
.
1. Addition of Hydrogen Halides to
Alkenes
In case of the reaction of
hydrogen halides with an alkene, hydrogen atom is attached to one of the
carbons around the double bond, halide is attached to the other carbon and
alkyl halide is obtained. This reaction is called electrophilic addition
reaction.
This kind of reactions occur in
compounds which have double or triple bonds between carbon atoms. The reaction
of hydrobromic acid with ethylene and 2-butene can be given as examples.
The mechanism of the reaction is
as follows:
1. Hydrobromic acid contains
positive proton (H+) and negative bromide (Br -) ions.
2. Proton is added to the double
bond of ethylene and carbonium ion forms.
3. Negative bromide ion adds to
positive carbonium ion and forms ethyl bro- mide (alkyl halide).
The steps of this mechanism apply
to all alkenes which are symmetrical around the double bond. Therefore, it does
not matter which carbon atom adds hydrogen and halide. The result is always the
same, the same compound is obtained.
But if the groups around the double
bond are not symmetrical, the reaction mechanism follows Markovnikov’s rule.
According to Markovnikov’s rule,
2-bromopropane is favored instead of 1-bro- mopropane.
Vladimir Markovnikov assigned the
following rule after so many experiments. When a compound is added to a double
bonded compound with asymmetric groups, the positive ion of the compound adds
to the carbon with greater num- ber of hydrogen atoms around the double bond.
Negative ion adds to the other carbon (with less hydrogen) of the double bond.
Tertiary (3°) carbonium ion the
most stable among the secondary (2°) and pri- mary (1°) carbonium ions, whereas
secondary carbonium is more stable than the primary one.
In the example above, Compound A
is more stable therefore it is formed more than Compound B.
Example 1
Prepare the compounds below:
1. ethyl chloride (chloroethane) from ethylene
2. -2iodopropane from propene
Solution:
Physical Properties of Alkyl Halides
Alkyl halides like CH3CH2Cl,
CH3Br, CH3Cl are in gas form at room tempera- ture. Alkyl halides up to C18 are
in liquid form and colorless. Alkyl halides that have more than 18 carbons are
colorless and solid. They do not dissolve in water but dissolve in organic
solvents. The reason for their water-insolubility is that they cannot form
hydrogen bonds with water.
Chemical Properties of Alkyl Halides
On the carbon which halogen is
attached, the bond between carbon and hydro- gen atoms is polarized as the
halogen of alkyl halides has more electrons than carbon. The poles of this bond
vary according to the type of halogen atom. For example, in alkyl iodide, the
polarization is very low. The carbon which has the halogen atom becomes the
nucleophile group. The most significant reactions of alkyl halides are
nucleophilic substitution reactions.
For this type of reactions, the following
can be given as examples:
A- The reaction of alkyl halides
with aqueous potassium hydroxide (KOH) solution
In this reaction, as shown in the
following equation, halogen atom substitutes with hydroxyl group (-OH) and
alcohol is formed.
B- The reaction of alkyl halides
with alcoholic potassium hydroxide (KOH)
When an alkyl halide reacts with
alcoholic KOH, an alkene is obtained. In this reaction, HX molecule is lost
from alkyl halide. This is a method of production of alkenes. For example:
C- The reaction of alkyl halide
with magnesium metal in dry ether
Alkyl halides react with Mg metal
to form methyl magnesium iodide (electrophile). For example:
References
1. K. J. Denniston c J. J.Topping c and R. L.Caretc
“General Organic and Biochemistry”c Mc-Graw- Hillc New York
(2004).
2. K.W. Whittenc R.E. Davis and L. M. Peckc “General Chemistry” 7th ed.
Holt Rinehart and Winstonc New York (2010).
3. Clayden, J.; Greeves, N. and Warren, S. (2012) Organic
Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.
4. Streitwieser, Andrew; Heathcock, Clayton H.; Kosower,
Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4:
Medtech (Scientific International, reprint of revised 4th edition, Macmillan,
1998). ISBN 978-93-85998-89-8.
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