CHAPTER 4 ALKYL HALIDE BY MAHWASH HAFEEZ

Alkyl Halides Alkyl halides have the general formula R-X R is alkyl group (functional group) and X is the halogen atom ( CL- Br-F) Alkyl groups are alkanes with one hydrogen removed. It is named by replacing the “-ane” of the alkane with “–yl”. Alkyl group + halides = alkyl halide.

Categories of alkyl halides R-X (X = F, Cl, Br, I) According to the class of the carbon that the halogen is attached to, alkyl halides are classified into three categories. R 3 C-X R 2 CH-X RCH 2 -X 2 o 3 o 1 o Primary secondary tertiary

1- Nomenclature (common name) Replace ( ane ) from alkane and addition ( yl ) and add name of halogen ( X ) Methane Methyl ethane ethyl propane propyl Hexane Hexyl Halide( halogen ) chlorideCl IodideI BromideBr FlouriedF

: (IUPAC)2- Nomenclature for Alkyl Halides 1.The longest continuous chain of carbon atoms containing the halogen atom is the parent. 2.Number the parent chain from the end nearest the halogen atom. CH 3 ׀ CH 3 – CH – CH 2 – Br 1-bromo-2-methylpropane

CH 3 I CH 2 I 2 iodomethane diiodomethane methyl iodide methylene iodide CHI 3 CI 4 triiodomethane tetraiodomethane iodoform carbon tetraiodide

8 Alkyl halides are weak polar molecules. They exhibit dipole-dipole interactions because of their polar C—X bond. Alkyl halide have higher boiling points than alkane with the same number of carbon Alkyl halides have relatively low melting points Alkyl halides are insoluble in water because of their in ability to form hydrogen bonding. Physical Properties

SYNTHESIS OF ALKYL HALIDE : 1-from alcohols: a) With hydrogen halide (HX): CH 3 CH 2 -OH + HI  CH 3 CH 2 -I + H 2 O ethyl alcohol ethyl iodide ethanol iodoethane Alcohol tend to under go rearraangement during replacement of OH by X b) With phosphorus halide (PX 3 ): CH 3 CH 3 | | CH 3 CHCH 2 -OH + PBr 3  CH 3 CHCH 2 -Br isobutyl alcohol isobutyl bromide 2-methyl-1-propanol 1-bromo-2-methylpropane

2.Halogenation of certain hydrocarbons: Alkanes react with halogens upon heating to form alkyl halides. R-H + X-X Δ or hν R-X + HX (requires Δ or hν; Cl 2 > Br 2 (I 2 NR); 3 o >2 o >1 o ) CH 3 CH 3 ׀ ׀ CH 3 – C – CH 3 + Cl 2 heat CH 3 – C – CH 2 – Cl ׀ CH 3 CH 3 2,2-dimethylpropane 1-chloro-2,2-dimethylpropane

CH 4 + Cl 2 hv CH 3 Cl + HCl methyl chloride (chloromethane) CH 3 Cl + Cl 2 hv CH 2 Cl 2 + HCl methylene chloride (dichloromethane) CH 2 Cl 2 + Cl 2 hv CHCl 3 + HCl chloroform (trichloromethane) CHCl 3 + Cl 2 hv CCl 4 + HCl carbon tetrachloride (tetrachloromethane) 3- Chlorination of methane

4- Addition of hydrogen halides to alkene: CH 2 =CH 2 + HCl → CH 3 -CH 2 -Cl Ethene Ethyl chloride 5- Addition of halides to alkane:

14 Alkyl halides are organic molecules containing a halogen atom bonded to an sp 3 hybridized carbon atom. 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. The halogen atom in halides is often denoted by the symbol “X”. Alkyl Halides and Nucleophilic Substitution

Nucleophilic Substitution A nucleophile can donate a pair of electrons to the electrophilic carbon atom of the alkyl halide forming a new bond and displacing the halide.

S N 1 reaction It involves two steps: * Slow step (rate determining): Ionization of the alkyl halide to form a carbocation(intermediate). *Fast step: Addition of the nucleophile to the carbocation. Rate = k[RX] First order reaction

17 MECHANISM: Bond breaking occurs before bond making.

EXAMPLE:

19 Mechanism: This first step involves an initial ionisation of the t- butyl bromide (2-bromo–2–methyl propane) forming a carbocation: Bond breaking step Slow step 2-bromo–2–methyl propane Carbocation The second step involves a very rapid attack by the hydroxide ion on the carbocation (carbonium ion) to form 2–methyl–2– Propanol: Bond forming + Na + step Fast step Carbocation 2–methyl–2– Propanol

SN2 Mechanism An SN2 reaction takes place in one step - attack of the nucleophile on the alkyl halide. This reaction does not have an intermediate. A transition state is formed in which the bond between the nuclophile and carbon atom is being formed at the same time the bond between the carbon atom and the leaving group is being broken. Rate = k[RX][Z - ]

Second order reaction

Reaction Of alkyl Halides 1. Dehydrohalogenation of alkyl halides: When heated with strong bases, alkyl halides typically undergo a 1,2- elimination reactions to generate alkenes. 2. Reaction with Metals: Alkyl halides react with metals to give Grignard reagent. CH 3 – Cl Mg CH 3 – Mg –Cl ether

3. Willaimson ether synthesis: An ether is prepared by the nucleophilic substitution (typically SN 2 reaction ) of an alkyl halide with an alkoxide ion. C 2 H 5 OH Na C 2 H 5 O – Na + C 2 H 5 Cl C 2 H 5 – O – C 2 H 5 NaCl Ethyl alcohol Alkoxide Di-ethyl ether 4. Hydrolysis: On hydrolysis alkyl halides yield alcohols. CH 3 – Br + NaOH H – OH CH 3 – OH + NaBr 5. Kolbe nitrile synthesis: Alkyl halides undergo nucleophilic substitution with alkali metal cyanides to form alkyl nitriles.

Uses of alkyl halides 1. Chloroflourocarbons are alkyl halides.e.g. Feron – 12. It was used as a coolant in refrigerators. However its use is now banned due to the damage this compound has caused to ozone layer. 2. 1,1,1-trichloroethane ( an alkyl halide) is used in making degreasing agents, glue, paint, and cleaning products.

3. Carbon tetrachloride is also an alkyl halide which is used in fire extinguishers, dry-cleaning businesses, and cleaning products. 4. Bromomethane is implemented in many pesticides as well as fumigants. As a fumigant it is used to help contain the spreading and overgrowth of certain fungi, nematodes, and weeds. 5. Chloroform is a clear liquid with an ether-like odor and a slightly sweet taste. It is used as anesthetic, to make coolants and as a dry cleaning spot remover.