Solutions-Class-12-Chemistry-Chapter-10-Halogen Derivatives-Maharashtra Board

(d) IV < III < I < II

Explanation:

Alkyl halides are more reactive than aryl halides. This is because of the partial double bond character between the benzene ring and the halogen. Further, in alkyl halides, the greater the stability of carbocation formed, the more would be the reactivity. Thus, the tertiary halide is more reactive. Nitro group which is electron-withdrawing increases the reactivity of aryl halide.

(b) CH₃-CH₂-CH₂I

(a) Only I

(b) Swartz reaction

(d) 2-Bromobutane

(d) n-propyl chloride with KCN

(c) 2-iodo-2 methylbutane

(b) chloroform

(a) 4-Bromo-3, 4 dimethyl but-2-ene

(b) 2-Chloro-3-methyl pentane

(c) 1-Chloro-4-ethyl cyclohexane

(d) 1,4-Dichloro-2-methyl benzene

(a) Since is a primary halide it undergoes S_N2 reaction faster than

(b) Since iodine is a better leaving group than chloride, 1-iodopropane (CH₃CH₂CH₂I) undergoes S_N2 reaction faster than 1-chloropropane (CH₃CH₂CH₂Cl).

2-Bromopropane < 1-Bromopropane < 1-Bromo-2-methylpropane < 1-Bromo butane.

• The low reactivity of aryl halides is due to resonance effect and sp2 hybrid state of carbon to which halogen atom is attached.
• In aryl halides, one of the lone pairs of electrons on halogen atom is in conjugation with π-electrons of the ring. Due to resonance, the C-X bond acquires partial double bond character. Thus, the C-X bond in haloarenes is stronger and shorter than haloalkanes. Hence, it is difficult to break C-X bond in haloarenes. (e.g. C-CI bond length in chlorobenzene is 169 pm as compared to C-CI bond length in alkyl chloride which is 178 pm).

Therefore, haloarenes are less reactive than haloalkanes.

• Alkyl halides cannot form hydrogen bonds with water.
• In addition to this, the attraction between alkyl halide molecules is stronger than the attraction between alkyl halide and water.

Hence, alkyl halides though moderately polar are immiscible with water.

• Grignard reagent (R Mg X) is an organometallic compound. The carbon-magnesium bond is highly polar and magnesium halogen bond is in ionic in nature. Grignard reagent is highly reactive.
• The reactions of Grignard reagent are carried out in dry conditions because traces of moisture may spoil the reaction and Grignard reagent reacts with water to produce alkane. Hence, reactions involving Grignard reagent must be carried out under anhydrous condition.

• Free radical halogenation of alkane gives a mixture of all different possible Monohaloalkanes as well as polyhalogen alkanes.
• In this method, by changing the quantity of halogen the desired product can be made to predominate over the other products. Hence, alkyl halides are generally not prepared by free radical halogenation of alkane.

• The stereoisomerism in which the isomers have different spatial arrangements of groups/atoms around a chiral atom is called optical isomerism.
• 2-chlorobutane has one chiral carbon atom. The spatial arrangement of the four different groups around the chiral atom is different.

• Two different arrangements of these groups around the carbon atom are possible as shown in the figure. Hence, it exists as a pair of enantiomers. The two enantiomers are mirror images of each other and are not superimposable.
• One of the enantiomers will rotate the plane of plane-polarized light to the left hand side and is called the laevorotatory isomer (l-isomer). The other enantiomer will rotate the plane of plane-polarized light to the right hand side and is called the dextrorotatory isomer (d-isomer).
• Equimolar mixture of the d- and the l-isomers is optically inactive and is called the racemic mixture or the racemate (dl-mixture). The optical inactivity of the racemic mixture is due to external compensation.

(a)

(b)

(1) Vinyl halide

(2) C-X bond length shorter in vinyl halide than alkyl halide. Vinyl halide has partial double bond character due to resonance.

In vinyl halide, carbon is sp²-hybridized. The bond is shorter and stronger and the molecule is more stable.

(3) Yes, It reacts by S_N1 mechanism. S_N1 mechanism involves formation of carbocation intermediate. The vinylic carbocation intermediate formed is resonance stabilized, hence S_N1 mechanism is favoured.