14.1 introduction to haloalkanes learning objectives: 1.be able to name halogenalkanes. 2.describe...

14.1 Introduction to Haloalkanes

Learning Objectives:1. Be able to name halogenalkanes.2. Describe the physical properties of

halogenalkanes.3. Explain the trend in bond polarity.4. Explain the trend in reactivity.

Task

Draw the skeletal formula and structural formula for the following:

- 1-iodopentane- 2-bromobutane- 3-chloropentane- 2-chloro-3-methylbutane

1-iodopentane

I

2-bromobutane

3-chloropentane

2-chloro-3-methylbutane

Cl

tetrachloromethane

iodomethane

dibromodifluoromethane

1,1-diiodoethane

1-chloro-3-fluoro-2-iodopropane

A chain of carbon atoms can be represented by R when drawing the structure. This is referred to as an R group.

Primary, secondary and tertiary

Primary (1°) halogenoalkanes have one R group attached to the carbon linked to the halogen.

Secondary (2°) halogenoalkanes have two R groups attached to the carbon linked to the halogen.

Tertiary (3°) halogenoalkanes have three R groups attached to the carbon linked to the halogen.

Physical Properties – Boiling Point• Explain the trend in boiling points of haloalkanes:

Why are fluorides

not on the chart?

Physical Properties – Boiling Pt.• Explain the trend in boiling points of haloalkanes:

Physical Properties – Solubility

• The carbon-halogen bond is polar (halogens more electronegative than carbon).

• But not polar enough to be soluble in water.

• Haloalkanes are…–soluble in organic solvents–insoluble in water

Bond Polarity

• Difference in electronegativity makes the C-X bond polar.

Carbon

Reactivity:Bond Polarity vs. Bond Enthalpy

• Two factors determine the reactivity of haloalkanes– Bond polarity– Bond enthalpy

Bond enthalpy is the more

important factor, reactivity increases DOWN the group.

14.2 Nucleophilic Substitution

Learning Objectives:1. Define the term nucleophile and name some

examples.2. Describe the nucleophilic substitution

reaction with haloalkanes.3. Draw the mechanism for a nucleophilic

substitution reaction.

Carbon-Halogen Bond is Polar

• The partial positive charge on the carbon atom (electron deficient) attracts species with a negative charge (electron rich).

Nucleophiles• Nucleophiles are electron rich and are attracted to

the partial positive charge on the carbon. Have lone pairs attached to an electronegative atom.

• Nucleophiles = electron pair donors

Examples:

OH¯ CN¯ NH3 H2O

δ-δ-

Nucleophilic Substitution Mechanism

1. Lone pair on nucleophile “attacks” the δ+ carbon and forms a dative covalent bond.

2. The halogen takes both electrons from the C-X bond and becomes the leaving group (X-).

“curly arrow” represent movement of electron pairs

AQUEOUS SODIUM HYDROXIDE

ANIMATED MECHANISM

NUCLEOPHILIC SUBSTITUTION

Nucleophilic Substitution – OH-

• Haloalkanes react with hydroxides to produce alcohols and halide ions.

Nucleophilic Substitution – CN-

• Haloalkanes react with cyanide ions to produce nitriles and halide ions.

Nucleophilic Substitution – NH3

• Haloalkanes react with ammonia to produce amines and halide ions.

1)

2)

14.3 Elimination Reactions

Learning Objectives:1. Describe the elimination reaction of

haloalkanes.2. Draw the mechanism for an elimination

reaction.3. Describe the conditions that would favor

elimination over substitution.

Elimination Mechanism1. OH- removes a H+ ion from the haloalkane to

form water.2. The electrons in the C-H bond now form a C-

C double bond.3. The C now has 5 bonds so the halogen takes

the electrons from the C-X bond and is eliminated as a halide ion.

ELIMINATION

ANIMATED MECHANISM

Mixture of elimination productsIf the carbon chain is four or more carbons in length and the halogen is not attached to a terminal carbon, a mixture of positional isomers may be formed.

attack at Aattack at BA B

but-2-ene but-1-ene

Conditions:Substitution vs. Elimination

Substitution• Cold hydroxide• Aqueous solution

• Primary haloalkanes

• OH- acts as a nucleophile

Elimination• Hot hydroxide• Ethanolic solution

(NO WATER)• Tertiary

haloalkanes• OH- acts as a base

(proton acceptor)

14.4 Formation of Haloalkanes

Learning Objectives:1. Define the term free radical.2. Describe the reaction mechanism for free-

radical substitutions.

Free Radical

• Free radical = species with an UNPAIRED electron

Cl.

Free Radical Substitution

1. Initiation (free radicals are formed)2. Propagation (free radicals react to

form additional free radicals)3. Termination (free radicals react and

are removed)

Initiation

• UV light provides the energy needed to break the Cl-Cl bond.

Propagation• Free radicals are EXTREMELY reactive. They

react with molecules to form new free radicals. This is a chain reaction.

Termination• When two free radicals react it removes the free

radicals and terminates the chain reaction. • There are multiple termination products.

Example: CFC’s and the Ozone Layer

• Chlorofluorocarbons (CFC’s) are haloalkanes.

• They are known to cause damage to the ozone layer (which protects us from harmful UV radiation).

• Ozone is destroyed through a free radical substitution reaction.

Read pg. 193 and 195 and make notes on CFCs

and the ozone layer including the chemical equations for the free

radical reactions.