LECTURE 4

Dr Ali El-Agamey

CHEM-103:

BASIC ORGANIC CHEMISTRY

DAMIETTA UNIVERSITY

Chapter 4 2

Reaction mechanism •  Overall reaction: reactants → products •  Mechanism: Step-by-step pathway.

Chapter 4 3

Chlorination of Methane

c

Chapter 4 4

Mechanism

c

Initiation Step

Chapter 4 5

Chlorination of Methane

•  Reaction requires heat or light for initiation. •  Many molecules of product are formed from

absorption of only one photon of light (chain reaction).

Chapter 4 6

Termination Steps

•  A reaction is classified as a termination step when any two free radicals join together producing a nonradical compound.

•  Combination of free radical with contaminant or collision with wall are also termination steps.

Chapter 4 7

More Termination Steps

Chapter 4 8

The Free-Radical Chain Reaction

•  Initiation generates a radical intermediate. •  Propagation: The intermediate reacts with a

stable molecule to produce another reactive intermediate (and a product molecule).

•  Termination: Side reactions that destroy the reactive intermediate.

Chapter 4 9

Energy Diagram for the Chlorination of Methane

(3) Halogenation

Other Examples

Chapter 4 11

Examples

•  Bromination of alkanes is much more selective than chlorination.1

•  What is the relative reacitivity of primary and secondary hydrogens in each reaction?

Chapter 4

12

Chlorination: Reactivities of Primary, Secondary, and Tertiary Hydrogens

(3) Halogenation

•  Secondary free radical is more stable than primary free radical.

14

Chlorination Energy Diagram Why is 2o H more reactive than 1o H?

•  Lower Ea (more stable T.S.), faster rate, so more stable intermediate is formed faster.

Chapter 4 15

Bromination: Relative reactivity of hydrogens

Chapter 4 16

Energy Diagram for the Bromination of Propane

Chapter 4 17

Tertiary hydrogen atoms react with Cl• about 5.5 times as fast as primary ones. Predict the product ratios for chlorination of isobutane.

Problem

Chapter 8 18

Write the mechanism (initiation and propagation) of the following reaction:1

Homework

Calculate the relative reactivity of different hydrogens in isobutane toward chlorination using the following equation:2

Alkyl Halides

©2010, Prentice Hall

Chapter 6 20

Alkyl Halides Classification •  Methyl halides: halide is attached to a methyl

group. •  Primary alkyl halide: carbon to which halogen

is bonded is attached to only one other carbon. •  Secondary alkyl halide : carbon to which

halogen is bonded is attached to two other carbons.

•  Tertiary alkyl halide : carbon to which halogen is bonded is attached to three other carbon.

Chapter 6 21

primary alkyl halide secondary alkyl halide

tertiary alkyl halide

Primary, Secondary, Tertiary Alkyl Halides

*

* *

Chapter 6 22

Types of Dihalides

•  Geminal dihalide: two halogen atoms are bonded to the same carbon.

•  Vicinal dihalide: two halogen atoms are bonded to adjacent carbons.

geminal dihalide

vicinal dihalide

Write the IUPAC names of the following compounds

Fluoroethane 1-Chlorobutane 2-Bromopropane

Iodocyclohexane

Homework: Write the IUPAC name of the following compounds

Homework: Write the structures for the following compounds

  (ii) 3-Bromo-2-methylpentane

  (iv) 2-Bromo-3-ethyl-2-methylhexane

  (v) 1,1-Dibromopropane

Preparation of Alkyl halides

Preparation of Alkyl halides

Chapter 6 27

Polarity and Reactivity

•  Halogens are more electronegative than C. •  Carbon—halogen bond is polar, so carbon has partial

positive charge. •  Carbon can be attacked by a nucleophile. •  Halogen can leave with the electron pair.

Chapter 6 28

•  The halogen atom on the alkyl halide is replaced with a nucleophile (Nuc-).

•  Since the halogen is more electronegative than carbon, the C—X bond breaks heterolytically and X- leaves.

Reactions of Alkyl halides (A) Substitution Reactions

Chapter 6 29

Reactions of Alkyl halides