Alkyl Halides 2024 PDF

# Alkyl Halides - Compounds in which halogen atom is bonded to sp3 hybridized carbon atom. - General formula is RX or CnH2n+1X (where X = Cl, Br, I or F) - Functional group is X. ## Classification ### Primary, Secondary or Tertiary Alkyl Halides: - Depends on whether the X atom is attached to pr...

# Alkyl Halides - Compounds in which halogen atom is bonded to sp3 hybridized carbon atom. - General formula is RX or CnH2n+1X (where X = Cl, Br, I or F) - Functional group is X. ## Classification ### Primary, Secondary or Tertiary Alkyl Halides: - Depends on whether the X atom is attached to primary (1°), secondary (2°) or tertiary (3°) carbon. | | Formula | Type | | :-: | :-: | :-: | | | CH3CH2CH2X | n-propyl halide | | | CH3CHXCH3 | sec-propyl halide | | | CH3-C-CH3 | ter-butyl halide | | | | | | | | | | | | | ## Comparing Halides - C-X bond is formed by overlapping of sp3 hybridized orbital of alkyl carbon with half-filled orbital of the halogen. | Bond| Hybridisation | |:---|:---| | CH3-F | 2sp3 - 2pz | | CH3-Cl | 2sp3 - 3p2 | | CH3-Br | 2sp3 - 4pz | | CH3-I | 2sp3 - 5pz | - Bond Energy Decreases - Bond Length Increases - Overlapping with same principle quantum number give stronger bond. - C-X bond is polar due to high electronegativity of X. - C atom of alkyl halides are positively polarized, RX are good electrophiles. ## Preparation of Alkyl Halides ### 1. From Alkanes: - Alkanes when react with Br2 in presence of UV light or at high temp obtain polysubstituted halides - Example: CH4 + UV -> CH3Cl + CH2Cl2 + CHCl3 + CCl4 ### 2. From Alkenes - Halogen acids (HCl, HBr, HI) add to alkenes to yield alkyl halides. - Markonikov's rule is followed during addition: CH3-CH=CH2 + HBr -> CH3-CHBr-CH3 - While in presence of organic peroxides anti-markonikov's rule is followed - Example: CH3-CH=CH2 + HBr -> CH3CH2CH2Br ### 3. From Alcohols - Alcohols when react with HBr or HI, alkyl bromides or iodides are obtained. - Example: CH3CH2OH + HBr -> CH3CH2Br + H2O ### 4. Other methods: - With Phosphorus halides (PCl5 or PCl3) - Example: 2CH3CH2OH + PCl5 -> 2CH3CH2Cl + POCl3 + H2O - With thionyl chloride (SOCl2) - Example: CH3CH2OH + SOCl2 -> CH3CH2Cl + SO2 + HCl ## Physical Properties: 1. CH3F, CH3Br, CH3Cl and CH3I are gases at room temperature. Alkyl halides up to C4 are liquids and beyond are solids. 2. Insoluble in water but soluble in organic solvents as they do not form hydrogen bonds with water. 3. Alkyl halides with large surface areas have large London attraction (temporary dipole) and thus higher bps. - Surface area increases as you go down the halide group: CH3F < CH3Cl < CH3Br < CH3I. - As branching increases, the R+ becomes more spherical, lower surface area and thus lower bp: n butyl Chloride > tert-butyl chloride (78°C > 52°C) ## Chemical Properties: - Alkyl Halides are very readily undergo substitution and elimination reactions. - They are very reactive compounds and undergo substitution reactions. ### Nucleophile: - Any reagent that donates an unshared pair of e-'s to form a new covalent bond. ### Nucleophilic substitution reaction: - Where Nucleophile is substituted for another. Nu: + R-X -> R-Nu + X- - All nucleophiles are bases, therefore B-elimination of halogens competes with substitution reaction: ### SN2 mechanism - Bimolecular Nucleophilic Substitution. - Takes place in one step. - Is a cConcerted mechanism. - Involves backside attack of by Nucleophile. - Both alkyl halide and Nucleophile are involved in RDS. - Inversion of Configuration occurs if the Nu attacks chiral centre. Example: :OH + CH3-Br -> CH3OH + Br- Mechanism: [ image: SN2 mechanism ] Rate = k[CH3Br][OH-] ### SN1 mechanism - Unimolecular Nucleophilic Substitution. - Takes place in two or more steps. - Results in the formation of racemic mixture if the substitution happens on chiral centre. - Only alkyl halide is involved in RDS. Example: CH3-C-Br + CH3OH -> CH3-C-OCH3 | | | | | | | | H CH3 H CH3 Mechanism: Step1: [ image: SN1 mechanism ] Step 2: [ image: SN1 mechanism ] ### Experimental evidence for the two contrasting mechanisms: Factors affecting Nucleophilic Substitution reactions: ### A. Structure of Nucleophile: - In case of SN2 reaction, rate determining step involves the reaction of Nu with alkyl halide. - Effectiveness of Nu is measured by kinetic study between an alkyl halide with different Nucleophiles. **Trend in Nucleophilicity is as follows:** | Group | Nucleophile | |:---------------|:---| | Br, I | :OH, :OR, :RS | | CH3S, RS | CH3SH, RSH, R2S, CH3OH, ROH | | H2O, CH3OH, RO | NH3, RNH2, R2NH, NH2-, RN2-, CH3O-, RO- | - In SN1 mechanism Nu do not participate in RDS, therefore Nu reacts at the same rate regardless of nucleophilicities. ### B. Structure of Alkyl Halide: - SN2 reactions are governed by steric factor. - Reaction is sensitive to crowding about the reaction site. - If you compare a Nu approaching 1°/2°/3° alkyl halide then ease of reaction will be in the order: 3° < 2° < 1° | | | | :-: | :-: | :-: | | CH3-C-Br | CH3 | H-C-Br | | | CH3 | CH3 | | | | | | | | | | | | H-C-Br | | | | CH3 | - Crowding of groups occur at reaction center and thus blocks the attack of approaching Nu from backside. - SN1 reaction proceeds via formation of Carbocation Intermediate. The stability of Carbocation is as follows: 1° < 2° < 3° - Thus: 1° alkyl halide are more likely to react with Nu than 2° and 1° are least active to undergo SN1 reaction: | | Stability | Nevel by SN2 | | :-: | :-: | :-: | | 1° | Low | High | | 2° | Moderate | Moderate | | 3° | High | Low | - Crowding decreases as you go up the alkyl halide group: ### C. Leaving group: - Leaving group develops partial -ve charge in both SN1 and SN2 reaction. - The best Leaving groups would be those most stable anions. - Conjugate bases of strong acids are best leaving groups: - I- > Br- > Cl- > F- - Mostly act as leaving groups: ### D. Solvent: - In SN1 reaction, ionization of C-X bond takes place at the RDS to give X anion and carbocation. - Protic solvents (e.g. H2O, low mol wt alcohols, low mol wt carboxylic acids) containing -CH groups are able to solvate both anionic and cationic species and thus are good solvents for SN1 reaction (eg. by H-bonding). - Aprotic solvents do not contain -OH groups and thus can not function as H-bond donors. e.g. DMSO (both polar) , dichloromethane, diethyl ether (both non-polar). They act as good solvents for SN2 reactions.