Nucleophilic Substitution and Alkyl Halides Essay

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Organic Chemistry, Fourth Edition
Janice Gorzynski Smith
University of Hawai’i

Chapter 7
Lecture Outline
Prepared by Layne A. Morsch
The University of Illinois - Springfield

Copyright © 2014 The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1

Alkyl Halides
• Alkyl halides are organic molecules containing a halogen atom bonded to an sp3 hybridized carbon atom.
• The halogen atom in halides is often denoted by the symbol
“X”.
• 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.

2

Types of Alkyl Halides
• Vinyl halides have a halogen atom (X) bonded to a C-C double bond.
• Aryl halides have a halogen atom bonded to a benzene ring. Figure 7.2

3

Types of Alkyl Halides
• Allylic halides have X bonded to the carbon atom adjacent to a C-C double bond.
• Benzylic halides have X bonded to the carbon atom adjacent to a benzene ring.
Figure 7.2

4

Naming Alkyl Halides

5

Problem 7.4
Give the IUPAC name for each compound.

Problem 7.5
Give the structure corresponding to each name.
3-chloro-2-methylhexane
cis-1,3-dichlorocyclopentane
1,1,3-tribromocyclohexane
6

Polarity of Alkyl Halides
• Alkyl halides are weakly polar molecules.
• They exhibit dipole-dipole interactions because of their polar C-X bond.
• Since the rest of the molecule contains only C-C and C-H bonds, they are incapable of intermolecular hydrogen bonding. 7

Physical Properties of Alkyl Halides

8

Simple Alkyl Halides
Figure 7.4

9

Common Alkyl Halides

10

The Polar Carbon-Halogen Bond
• The electronegative halogen atom in alkyl halides creates a polar C-X bond, making the carbon atom electron deficient.
• Electrostatic potential maps of four simple alkyl halides illustrate this point.
• This electron deficient carbon is a key site in the reactivity of alkyl halides.
Figure 7.5

11

Reaction Types for Alkyl Halides

12

Substitution Reactions
• Three components are necessary in any substitution reaction.

13

Nucleophiles in Substitution Reactions
• Nucleophiles are Lewis bases that can be negatively charged or neutral.
• Negatively charged nucleophiles like HO¯ and HS¯ are used as salts with Li+, Na+, or K+ counterions to balance the charge.
• Since the identity of the counterion is usually inconsequential, it is often omitted from the chemical equation.

14

Neutral Nucleophiles
• When a neutral nucleophile is used, the substitution product bears a positive charge.

• The substitution product’s positive charge is usually caused by a proton bonded to O or N.
• That proton is readily lost from this in a BrØnstedLowry acid-base reaction, forming a neutral product.

15

Drawing Products of Nucleophilic
Substitution Reactions

• The overall effect of any nucleophilic substitution is the replacement of the leaving group by the nucleophile.
• To draw any nucleophilic substitution product:
• Find the sp3 hybridized carbon with the leaving group.
• Identify the nucleophile, the species with a lone pair or π bond. • Substitute the nucleophile for the leaving group and assign charges (if necessary) to any atom that is involved in bond breaking or bond formation.
16

Problem 7.8
Identify the nucleophile and leaving group and draw the products of each substitution reaction.

17

The Leaving Group
• In a nucleophilic substitution reaction of R-X, the C-X bond is heterolytically cleaved, and the leaving group departs with the electron pair in that bond, forming X:¯.
• The more stable the leaving group X:¯, the better able it is to accept an electron pair.

• For example, H2O is a better leaving group than HO¯ because
H2O is a weaker base.
18

Trends in Leaving Group Ability
• The weaker the base, the better the leaving group.

19

Good Leaving Groups

20

Poor Leaving Groups
• Conjugate bases of weaker acids are poorer leaving groups. 21

Nucleophiles and Bases
• Nucleophiles and
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