Alkyl Halides notes

MDCAT Chemistry

Alkyl halides are derivatives of alkanes in which a halogen atom replaces a hydrogen atom. This chapter covers their nomenclature, structure, preparation, reactivity, nucleophilic substitution, Grignard reagents, and elimination reactions.

Definition, Classification and Nomenclature

Alkyl halides, also called haloalkanes, contain a halogen atom bonded to an sp3-hybridised carbon atom. Their general formula is R-X, where R is an alkyl group and X is F, Cl, Br or I. The carbon-halogen bond is polar because halogens are more electronegative than carbon.

Alkyl halides are named by selecting the longest carbon chain, numbering it from the end nearest the halogen or substituent, and using fluoro, chloro, bromo or iodo as prefixes. The parent hydrocarbon name is obtained from the longest chain containing the carbon attached to the halogen.

General formula of an alkyl halide: R-X.
Halogens in alkyl halides are fluorine, chlorine, bromine and iodine.
Methyl halide: CH3-X. Primary halide: R-CH2-X.
Secondary halide: R2CH-X. Tertiary halide: R3C-X.
(CH3)3CCH2Br is named 1-bromo-2,2-dimethylpropane.
CH3CH2CH2Cl is 1-chloropropane, while CH3CHClCH3 is 2-chloropropane.
In IUPAC nomenclature, fluoro, chloro, bromo and iodo are used as prefixes.

Structure and Reactivity of the Carbon-Halogen Bond

The carbon atom bonded to the halogen is sp3 hybridised and has approximately tetrahedral geometry. The carbon-halogen bond is polar, with carbon carrying a partial positive charge and the halogen carrying a partial negative charge. Therefore, the carbon atom can be attacked by electron-rich species called nucleophiles.

The carbon-halogen bond becomes weaker as the size of the halogen increases. Iodide is the best leaving group among the halides, while fluoride is the poorest leaving group in ordinary nucleophilic substitution reactions.

Bond strength generally follows C-F > C-Cl > C-Br > C-I.
Leaving group ability generally follows I- > Br- > Cl- > F-.
For the same alkyl group, reactivity follows R-I > R-Br > R-Cl > R-F.
A nucleophile attacks the electron-deficient carbon atom bonded to the halogen.
Primary, secondary and tertiary classification depends on the number of carbon groups attached to the carbon bearing the halogen.
Steric hindrance around the carbon-halogen bond decreases the rate of SN2 reaction.
Tertiary alkyl halides are usually more reactive in SN1 reactions because they form more stable carbocations.

Preparation of Alkyl Halides

Alkyl halides can be prepared from alkanes, alkenes and alcohols. Direct halogenation of an alkane occurs by a free-radical substitution mechanism. Chlorination and bromination are commonly used, but direct iodination is not normally suitable because the reaction is reversible and does not proceed effectively.

Alcohols are converted into alkyl halides using suitable halogenating reagents. Alkyl halides can also be produced by the addition of hydrogen halides to alkenes. The product may follow Markovnikov's rule, depending on the reaction conditions.

Direct halogenation of alkanes commonly uses Cl2 or Br2 in the presence of light or heat.
Alkyl iodides cannot normally be prepared effectively by direct iodination of alkanes.
Alcohols can be converted into alkyl halides using reagents such as HCl, HBr, HI, PCl3, PCl5 or SOCl2, depending on the alcohol and conditions.
Addition of HX to an alkene generally gives the Markovnikov product in the absence of peroxide conditions.
Addition of bromine to ethene forms 1,2-dibromoethane through a cyclic bromonium ion intermediate.
The intermediate in bromine addition to ethene is not a free carbocation.

Nucleophiles and Nucleophilic Substitution

A nucleophile is an electron-rich species that donates an electron pair to an electron-deficient atom. Nucleophiles may be negatively charged ions or neutral molecules having lone pairs. They attack the carbon atom of an alkyl halide and replace the halide ion.

Nucleophilic substitution is represented generally as R-X + Nu- → R-Nu + X-. The two main mechanisms are SN1 and SN2. SN means nucleophilic substitution, while the number shows the order of the rate-determining step.

Species produced by heterolytic bond breaking and able to donate an electron pair are called nucleophiles.
Examples of nucleophiles include OH-, CN-, I-, NH3, H2O and RO-.
The halide ion leaves as the leaving group during substitution.
A primary alkyl halide generally undergoes SN2 reaction.
A tertiary alkyl halide generally undergoes SN1 reaction in a suitable polar protic solvent.
Methyl halides are especially suitable for SN2 reactions because they have very little steric hindrance.
Nucleophilic substitution can produce alcohols, ethers, nitriles, amines and other organic compounds.

SN2 Mechanism and Reactivity

SN2 is a one-step, bimolecular nucleophilic substitution. The nucleophile attacks the carbon atom from the side opposite to the leaving group while the carbon-halogen bond breaks at the same time. There is no intermediate carbocation.

The rate depends on the concentration of both the alkyl halide and the nucleophile. Backside attack causes inversion of configuration at the carbon atom. This stereochemical change is called Walden inversion.

SN2 rate law: rate = k[alkyl halide][nucleophile].
SN2 is a single-step concerted reaction.
SN2 produces inversion in configuration.
SN2 is favoured by methyl and primary alkyl halides.
The order of SN2 reactivity due to steric effects is methyl > primary > secondary >> tertiary.
For the same alkyl group, SN2 reactivity follows R-I > R-Br > R-Cl > R-F.
The stated order for the given compounds toward SN2 displacement is III < I < IV < II, because compound III is tertiary, I and IV differ in primary or secondary structure and leaving group, and II has a primary carbon bonded to bromine.
SN2 reactions are favoured in the gas phase because solvent molecules do not surround and strongly solvate the reacting ions. In solution, polar aprotic solvents generally favour SN2 reactions.

SN1 Mechanism and Factors Affecting It

SN1 is a two-step, unimolecular nucleophilic substitution. The first and slow step is ionisation of the carbon-halogen bond. A carbocation and a halide ion are formed. In the second step, the nucleophile attacks the carbocation.

Because the carbocation is planar, attack may occur from either side. Therefore, an optically active substrate may produce a mixture containing retention and inversion of configuration, often with partial or considerable racemisation.

The first step in an SN1 reaction is ionisation.
The intermediate species in SN1 is a carbocation.
SN1 rate law: rate = k[alkyl halide].
Carbocation stability follows tertiary > secondary > primary > methyl.
SN1 is favoured by tertiary alkyl halides and substrates that form resonance-stabilised carbocations.
Polar protic solvents such as water and alcohol favour SN1 reactions because they stabilise ions and the transition state.
SN1 reactions may involve carbocation rearrangements such as hydride or alkyl shifts.
SN1 generally gives a mixture of stereochemical products because the carbocation is planar.

Grignard Reagents and Related Reactions

A Grignard reagent is an organomagnesium compound with the general formula RMgX, where R is an alkyl or aryl group and X is a halogen. It is prepared by reacting an alkyl halide with magnesium in dry ether. Water must be excluded because Grignard reagents react rapidly with water.

Grignard reagents behave as strong nucleophiles and strong bases. They add to carbonyl compounds and, after hydrolysis, produce alcohols. Their reaction with ethylene oxide gives a primary alcohol containing two more carbon atoms than the original alkyl group.

General formula of a Grignard reagent: RMgX.
Preparation: R-X + Mg → R-MgX in dry ether.
Grignard reagents must be protected from water and moisture.
RMgX + H2O gives RH, so water destroys the reagent.
Reaction with formaldehyde followed by hydrolysis gives a primary alcohol.
Reaction with an epoxide followed by hydrolysis gives a primary alcohol and lengthens the carbon chain.
Reaction with an aldehyde generally gives a secondary alcohol after hydrolysis.
Reaction with a ketone generally gives a tertiary alcohol after hydrolysis.

Nitriles, Friedel-Crafts Reaction and Amide Hydrolysis

Alkyl halides react with potassium cyanide to form nitriles. This reaction is useful in organic synthesis because the carbon of the cyanide group becomes part of the product. Thus, the carbon chain is lengthened by one carbon atom.

Some related reactions of aromatic compounds and amides are also connected with the chemistry of halogen compounds. Aluminium chloride acts as a Lewis acid catalyst in Friedel-Crafts alkylation and acylation. Acid hydrolysis of an amide produces a carboxylic acid and an amine or ammonia, depending on the amide structure.

R-X + KCN → R-CN + KX.
Reaction with KCN lengthens the carbon chain by one carbon atom.
The nitrile group is -C≡N.
Friedel-Crafts reactions use anhydrous AlCl3 as a Lewis acid catalyst.
Nitration of benzene uses concentrated HNO3 and concentrated H2SO4, usually at about 50 °C.
Acid hydrolysis of N,N-dimethylacetamide, (CH3)2NCOCH3, gives (CH3)2NH and CH3COOH.
KCN is mainly covalent in organic solvents and supplies the carbon-containing CN group.

Elimination Reactions and E2 Mechanism

Elimination removes a halide ion and a hydrogen atom from adjacent carbon atoms to form an alkene. Dehydrohalogenation of an alkyl halide is commonly carried out with alcoholic KOH and heat. The major product usually follows Saytzeff's rule, in which the more substituted alkene is formed.

E2 elimination occurs in one concerted step. A strong base removes a beta hydrogen while the leaving group leaves from the adjacent alpha carbon. The rate depends on both the alkyl halide and the base.

The carbon bearing the halogen is called the alpha carbon.
A carbon adjacent to the alpha carbon is called a beta carbon.
E2 rate law: rate = k[alkyl halide][base].
E2 is favoured by a strong base, heat and a suitable alkyl halide.
In a nonpolar solvent, addition of a strong base generally favours E2 elimination.
Tertiary alkyl halides commonly undergo E2 rapidly because substitution by SN2 is sterically hindered.
Alcoholic KOH generally gives an alkene by elimination, not an alcohol.
Saytzeff's rule states that the major alkene usually has the greater number of alkyl groups attached to the double-bonded carbons.

E1 Mechanism and Comparison of Reactions

E1 elimination occurs in two steps. The leaving group first ionises to form a carbocation, and then a base removes a beta hydrogen to form an alkene. Since carbocation formation is the slow step, the rate depends only on the concentration of the alkyl halide.

The mechanism selected depends on the substrate, nucleophile or base, solvent, temperature and steric effects. Substitution is favoured by a good nucleophile, while elimination is favoured by a strong base and increased temperature.

E1 rate law: rate = k[alkyl halide].
The first step of E1 is ionisation and carbocation formation.
E1 is favoured by tertiary substrates, polar protic solvents and heat.
E1 may involve carbocation rearrangement.
SN1 and E1 both form carbocation intermediates and are favoured by tertiary alkyl halides.
SN2 is one-step and gives inversion, whereas SN1 is two-step and may give racemisation.
E2 is one-step, while E1 is two-step.
Aqueous KOH commonly favours substitution to form an alcohol, whereas alcoholic KOH and heat commonly favour elimination to form an alkene.

Key terms

Alkyl halide: An organic compound in which a halogen is bonded to an sp3-hybridised carbon atom.
Leaving group: An atom or ion that departs with the bonding electron pair during a substitution or elimination reaction.
Nucleophile: An electron-rich species that donates an electron pair to an electron-deficient atom.
Carbocation: A positively charged carbon species having only six electrons around the positively charged carbon.
SN1 reaction: A unimolecular nucleophilic substitution involving carbocation formation in a slow first step.
SN2 reaction: A bimolecular nucleophilic substitution that occurs in one concerted step.
E1 reaction: A unimolecular elimination involving carbocation formation before loss of a beta hydrogen.
E2 reaction: A bimolecular elimination in which base removal and leaving-group departure occur simultaneously.
Grignard reagent: An organomagnesium compound with the general formula RMgX.
Walden inversion: The inversion of configuration caused by backside attack in an SN2 reaction.
Ionisation: Heterolytic breaking of the carbon-halogen bond to form a carbocation and a halide ion.
Dehydrohalogenation: Removal of hydrogen halide from an alkyl halide to form an alkene.
Saytzeff's rule: The rule that the more substituted alkene is usually the major product of elimination.
Bromonium ion: A three-membered cyclic intermediate formed when bromine adds to an alkene.
Nitrile: An organic compound containing the functional group -C≡N.

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