chapter 8 alkyl halides

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8. Alkyl Halides Dr Wong Yau Hsiung CHEM 221

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Page 1: Chapter 8 Alkyl  halides

8. Alkyl Halides

Dr Wong Yau Hsiung CHEM 221

Page 2: Chapter 8 Alkyl  halides

McMurry Organic Chemistry 6th edition, Chapter 10 (c) 2003

2

What Is an Alkyl Halide/ Haloalkanes An organic compound containing at least one carbon-

halogen bond (C-X) X (F, Cl, Br, I) replaces H

Can contain many C-X bonds Properties and some uses

Fire-resistant solvents Refrigerants Pharmaceuticals and precursors

Page 3: Chapter 8 Alkyl  halides

• Halogenoalkanes are similar to alkanes but with one or more of the hydrogen atoms replaced by a halogen.

• Halogenoalkanes can contain more than one type of halogen. For example, CFCs (chlorofluorocarbons) contain both chlorine and fluorine atoms. chloro-pentafluoroethane

trichloromethane

What are Haloalkanes?

Page 4: Chapter 8 Alkyl  halides

• Name is based on longest carbon chain (contains double or triple bond if present)

• Number the carbons of the parent chain beginning at the end nearer the first substituent, whether alkyl or halo

• 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

Naming Haloalkane

Page 5: Chapter 8 Alkyl  halides

• 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

Naming Haloalkane

Page 6: Chapter 8 Alkyl  halides
Page 7: Chapter 8 Alkyl  halides

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

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.

Primary, secondary and tertiary

R-CH2-X

R2-CH-X

R3-C-X

Page 8: Chapter 8 Alkyl  halides
Page 9: Chapter 8 Alkyl  halides

Physical Properties Solubility : All organic halides are insoluble in water and soluble in common organic solvents.Boiling point : The boiling points increases with increasing in

molecular weights. Therefore, the boiling points increases in the order F<Cl<Br<I.

BP also increases for “straight” chain isomers. Greater branching = lower BP

Page 10: Chapter 8 Alkyl  halides

How are halogenoalkanes made?

Page 11: Chapter 8 Alkyl  halides

There are several ways by which halogenoalkanes can be made, including: free radical substitution of an alkane:

electrophilic addition of HX or X2 to an alkene:

CH4 + Cl2 CH3Cl + HCl

C2H4 + HBr C2H5Br

C2H4 + Br2 C2H4Br2

How are halogenoalkanes made?

Page 12: Chapter 8 Alkyl  halides
Page 13: Chapter 8 Alkyl  halides

• The carbon–halogen bond in halogenoalkanes is polar because all halogens are more electronegative than carbon.

• The polar bond means that the carbon atom has a small positive charge (δ+), which attracts substances with a lone pair of electrons. These are nucleophiles, meaning ‘nucleus (positive charge) loving’. Examples include:

δ+ δ- δ+ δ- δ+ δ- δ+ δ-

ammonia cyanide hydroxide

Polar bonds and nucleophiles

Page 14: Chapter 8 Alkyl  halides

• The nucleophile uses its lone pair to provide the electrons for a new bond

• The halogen is displaced • The result is substitution following attack by a nucleophile

Nucleophilic Substitution

• The general form for the reaction is Nu:- + R-X R-Nu + X:

Nucleophile Substrate Product Leaving group

Page 15: Chapter 8 Alkyl  halides

Reagent Aqueous sodium (or potassium) hydroxide

Conditions Reflux in aqueous solution (SOLVENT IS IMPORTANT) Elimination takes place when ethanol is the solvent

The reaction with water is known as HYDROLYSIS

Product Alcohol

Nucleophile hydroxide ion (OH¯)

Equation C2H5Br(l) + NaOH(aq) —> C2H5OH(l) + NaBr(aq)

Nucleophilic Substitution – Aqueous NaOH

Page 16: Chapter 8 Alkyl  halides
Page 17: Chapter 8 Alkyl  halides
Page 18: Chapter 8 Alkyl  halides

Nucleophilic Substitution Bimolecular or SN2• The rate depends on the conc. of 2 reactants: the substrate and

the nucleophile. • Takes place in one step • No carbocation • new bond forming and old bond breaking at same time.• Occur most readily with methyl compounds and primary

haloalkanes

H3CH2C

C

CH3H

BrOH-

H3CH2C

C

CH3H

BrOH

CH2CH3

C

CH3H

OH

Transition State: As OH- attaches, Br- leaves

+ Br-+

Page 19: Chapter 8 Alkyl  halides

Nucleophilic Substitution Bimolecular or SN2• Transition state is highest in energy.

Page 20: Chapter 8 Alkyl  halides

Nucleophilic Substitution Unimolecular or SN1• The rate depends on the conc. of 1 reactant: the substrate but not

the nucleophile.• A two step process since the substrate and the nucleophile cannot

both appear in the rate determining step• Form carbocation intermediate• Occur most readily with tertiary haloalkanes and some secondary

haloalkanes.

H3CH2C

C

CH3H

Br

H3CH2C

C+

CH3H

CH2CH3

C

CH3H

OH

Transition State: Formation of Carbocation

Br-OH-

+

Rate determining step: spotaneous dissociation of leaving group

Very fast step: reaction of nucelophile and carbocation

Page 21: Chapter 8 Alkyl  halides

Nucleophilic Substitution Unimolecular or SN1

Page 22: Chapter 8 Alkyl  halides

SN1 and SN2

SN1 SN2

Rate =k[RX] =k[RX][Nuc:-]

Carbocation intermediate?

Yes No

Number of steps 2 1

Occurs with Tertiary halogenoalkanes

Primary halogenoalkanes

Page 23: Chapter 8 Alkyl  halides

• In the reaction with a strong base, halogenoalkanes will undergo not only nucleophilic substitution but also elimination reactions, forming alkenes and water.

• The OH- acts as both a base and a nucleophile. When acting as a base, the OH- removes H+ from the halogenoalkane, which also results in the formation of a halide ion.

• The reaction between a halogenoalkane and a strong base usually results in the formation of a mixture of substitution and elimination products.

Elimination in Haloalkanes

Page 24: Chapter 8 Alkyl  halides

Reagent Alcoholic sodium (or potassium) hydroxide

Conditions Reflux in alcoholic solution

Product Alkene

Mechanism Elimination

Equation C3H7Br + NaOH(alc) —> C3H6 + H2O + NaBr

Complication With unsymmetrical halogenoalkanes, you can get mixture of products

Elimination

Page 25: Chapter 8 Alkyl  halides

• If 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

Mixture of elimination products

can exist as cis and trans isomers

Page 26: Chapter 8 Alkyl  halides

Zaitsev’s Rule for Elimination Reactions • In the elimination of HX from an alkyl halide, the more highly

substituted alkene product predominates • If more than one elimination product is possible, the most-

substituted alkene is the major product (most stable).• R2C=CR2 > R2C=CHR > RHC=CHR > H2C=CHR

tetra > tri > di > mono

Page 27: Chapter 8 Alkyl  halides
Page 28: Chapter 8 Alkyl  halides

The products of reactions between haloalkanes and OH¯ are influenced by the solvent

SOLVENT ROLE OF OH– MECHANISM PRODUCT

WATER NUCLEOPHILE SUBSTITUTION ALCOHOL

ALCOHOL BASE ELIMINATION ALKENE

Modes of attack

Aqueous solution •OH¯ attacks the slightly positive carbon bonded to the halogen.•OH¯ acts as a nucleophile Alcoholic solution •OH¯ attacks one of the hydrogen atoms on a carbon atom adjacent the carbon bonded to the halogen.•OH¯ acts as a base (PROTON ACCEPTOR)

Both reactions take place at the same time but by varying the solvent you can influence which mechanism dominates.

Elimination vs Substitution

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