Organic Chemistry

M. R. Naimi-Jamal

Faculty of Chemistry

Iran University of Science & Technology

Chapter 7-1. Alkyl Halides

Based on McMurry’s Organic Chemistry, 6th edition

3

What Is an Alkyl Halide?

An organic compound containing at least one

halogen attached to an sp3 hybridized carbon

X (F, Cl, Br, I) replaces H

Can contain many C-X bonds

Properties and some uses

Fire-resistant solvents

Refrigerants

Pharmaceuticals and precursors

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Naming Alkyl Halides

Name is based on longest carbon chain

(Contains double or triple bond if present)

Number from end nearest any substituent

(alkyl or halogen)

Halogens have same priority as alkyl

groups

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Naming with Multiple Halides

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

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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

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Many Alkyl Halides That Are Widely Used Have Common Names

Chloroform CHCl3

Carbon tetrachloride CCl4

Methylene chloride CH2Cl2

Methyl iodide CH3I

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Properties of the Halomethanes

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Preparing Alkyl Halides

Alkyl halide is formed by the addition of HCl, HBr,

HI to alkenes to give the Markovnikov product

(see Alkenes chapter)

Alkyl geminal dihalide from anti addition of

bromine or chlorine

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Reaction of Alkanes with Halogens

Alkane + Cl2 or Br2, initiated by heat or light;

replaces C-H with C-X

Hard to control for monosubstitution

Reacts via a free radical mechanism

It is usually not a good idea to plan a

synthesis that uses this method

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Multiple Substitution

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Radical Halogenation of Alkanes

If there is more than one type of hydrogen in

an alkane, the reaction favors replacing the

hydrogen at the most highly substituted

carbons: 3o>2o>1o

The number of available hydrogens is a factor

Methyl hydrogens, for example, often

outnumber 2o or 3o hydrogens.

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Chlorination is unselective:

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Relative Reactivity

Based on quantitative analysis of reaction

products, relative reactivity is estimated

Order parallels stability of radicals

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Toward chlorination

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Bromination is much more selective

35% 65%

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Preparing Alkyl Halides from Alcohols

Reaction of tertiary C-OH with HX is fast and

effective

Add HCl or HBr gas into ether solution of tertiary

alcohol

Primary and secondary alcohols react very slowly and

often rearrange, so alternative methods are used

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Reactivity of Alcohols with HX

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Preparation of Alkyl Halides from Primary and Secondary Alcohols

Specific reagents avoid acid and

rearrangements of carbon skeleton

Thionyl chloride converts alcohols into alkyl

chlorides (SOCl2: ROH to RCl)

Phosphorus tribromide converts alcohols into

alkyl bromides (PBr3: ROH to RBr)

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Problem: Products?

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Prob.: Synthesize from alcohols?

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Reactions of Alkyl Halides: Grignard Reagents

Reaction of RX with Mg in ether or THF

Product is RMgX – an organometallic compound

(alkyl-metal bond)

R is alkyl 1°, 2°, 3°, aryl, alkenyl

X = Cl, Br, I

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François Auguste Victor Grignard

• Born in Cherbourg,

1871

• PhD, University of

Lyon, 1901

• Nobel Prize in

Chemistry, 1912

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Grignard Carbon as Nucleophile:

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Reactions of Grignard Reagents

Many useful reactions

RMgX behaves as R- (adds to C=O)

RMgX + H3O+ yields R-H

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Organometallic Coupling Reactions

Alkyllithium (RLi) forms

from RBr and Li metal

RLi reacts with copper

iodide to give lithium

dialkylcopper (Gilman

reagents)

Henry Gilman (1893-1986)

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Utility of Organometallic Coupling in Synthesis

Coupling of two organometallic molecules produces larger molecules of defined structure

Aryl and vinyl organometallics also effective

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Lithium dialkylcopper reagents react with alkyl halides to give alkanes:

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Problem:

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Problem: Identify reagents

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Problem

a = BH3/THF followed by H2O2/OH-

b = PBr3/ether

c = (CH3)2CuLi/ether

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Chapter 7-1, Questions

22, 23, 24, 26, 28