ALKYL HALIDES
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) In tertiary alkyl halides the halogen atom is attached to a carbon which is further attached to ______ carbon atoms directly. (three)
(ii) The best method for the preparation of alkyl halides is the reaction of ______ with inorganic reagents. (alcohol)
(iii) An alkyl group with a partial positive charge on the carbon atom is called _______ center. (electrophilic)
(iv) The mechanism is called _______ if it involves one molecule in the rate determining step. (unimolecular)
(v) Molecularity of a reaction is defined as the number of molecules taking part in the _______. (rate determining step)
(vi) The molecularity of E2 reactions is always two and the reactions show ________ order kinetics. (second)
(vii) Wurtz synthesis is useful for the preparation of _________ alkanes. (symmetrical)
(viii) Grignard reagents are prepared by the reaction of magnesium metal with alkyl halides in the presence of _________. (dry ether)
TRUE/FALSE
Q.02: Indicate True or False:
(i) In secondary alkyl halides, the halogen atom is attached to a carbon which is further attached to two carbon atoms directly. (TRUE)
(ii) Alcohols react with thionyl chloride in ether as solvent to give alkyl halides. (FALSE)
CORRECT: Alcohols react with thionyl chloride in pyridine as solvent to give alkyl halides.
(iii) Order of reactivity of alkyl halides for a particular group is: Iodide>Bromide>Chloride>Fluoride alkyl. (TRUE)
(iv) In SN2 reactions, the attacking nucleophile always attacks from the side in which the leaving group is attached. (FALSE)
CORRECT: In SN2 reactions. the attacking nucleophile always attacks from opposite side from which the leaving group is attached.
(v) Methyl magnesium iodide on hydrolysis yields ethyl alcohol. (FALSE)
CORRECT: Methyl magnesium iodide on hydrolysis yields methyl alcohol.
(vi) Primary, secondary and tertiary amines react with Grignard reagents in the same way. (FALSE)
CORRECT: Primary and secondary amines react with Grignard reagents in different way from tertiary amines.
(vii) The reactions of secondary alkyl halides may follow both SN1 and SN2 mechanisms. (TRUE)
(viii) SN1 mechanism is a one stage process involving a simultaneous bond breakage and bond formation. (FALSE)
CORRECT: SN2 mechanism is a one stage process involving a simultaneous bond breakage and bond formation.
(ix) In b-elimination reactions, the two atoms or groups attached to two adjacent carbon atoms are lost under the influence of an electrophile. (FALSE)
CORRECT: In b-elimination reactions, the two atoms or groups attached to two adjacent carbon atoms are lost under the influence of a nucleophile.
(x) The reactivity order of alkyl halides is determined by the strength of carbon- halogen bond. (TRUE)
MCQs
Q.03: Multiple choice questions.
(i) In primary alkyl halides, the halogen atom is attached to a carbon which is further attached to how many carbon atoms?
(a) Two
(b) Three
(c) Four
(d) One
ANSWER: (d) One
EXPLANATION: A carbon which is further attached to one or no carbon is called primary carbon, and if halogen atom is attached to the primary carbon, the alkyl halide will be called primary alkyl halide.
(ii) The reactivity order of alkyl halides for a particular alkyl group is:
(a) Fluoride>Chloride>Bromide>Iodide
(b) Chloride>Bromide>Fluoride>Iodide
(c) Iodide>Bromide>Chloride>Fluoride
(d) Bromide>Iodide>Chloride>Fluoride
ANSWER: (c) Iodide>Bromide>Chloride>Fluoride
EXPLANATION: The greater is the size of the halogen atom, the lower will be the bond energy, and as a result, greater will be the reactivity of alkyl halide.
(iii) When CO2 is made to react with ethyl magnesium iodide, followed by acid hydrolysis, the product formed is:
(a) Propane
(b) Propanoic acid
(c) Propanal
(d) Propanol
ANSWER: (b) Propanoic acid
EXPLANATION: The reaction of CO2 with ethyl magnesium iodide, the Grignard reagent, followed by acid hydrolysis, finally results in the carboxylation of ethyl group, giving three carbon carboxylic acid, the propanoic acid
(iv) Grignard reagent is reactive due to:
(a) The presence of halogen atom
(b) The presence of Mg atom
(c) The polarity of C — Mg bond
(d) None of the above
ANSWER: (c) The polarity of C — Mg bond
EXPLANATION: Magnesium is more electropositive than carbon and the C—Mg bond though covalent is highly polar, giving alkyl carbon the partial negative charge. This negative charge is an unusual character which makes the alkyl groups highly reactive towards electrophilic centers.
(v) SN2 reactions can be best carried out with:
(a) Primary alkyl halides
(b) Secondary alkyl halides
(c) Tertiary alkyl halides
(d) All of the three
ANSWER: (a) Primary alkyl halides
EXPLANATION: SN2 reactions are bimolecular reactions in which the attack of attacking nucleophile and the departure of the leaving group take place simultaneously in one step. It means the attacking nucleophile comes from the opposite direction of the leaving group, and this is best possible only in primary alkyl halides because due to the less number of adjacent carbons, it poses very low stearic hindrance, thus making the attack of the incoming nucleophile easier.
(vi) Elimination bimolecular reactions involve:
(a) First order kinetics
(b) Second order kinetics
(c) Third order kinetics
(d) Zero order kinetics
ANSWER: (b) Second order kinetics
EXPLANATION: Being bimolecular shows that the molecularity of these reactions is two which means two molecules are involved in the rate determining step. They are, therefore, single step reactions, following second order kinetics.
(vii) For which mechanisms, the first step involved is the same:
(a) E1 and E2
(b) E2 and SN2
(c) SN1 and E2
(d) E1 and SN1
ANSWER: (d) E1 and SN1
EXPLANATION: In both E1 and SN1 reactions, the first step is the reversible ionization of alkyl halide in the presence of an aqueous acetone or aqueous ethyl alcohol, resulting in the formation of a carbocation.
(viii) Alkyl halides are considered to be very reactive compounds towards nucleophiles because:
(a) They have an electrophilic carbon
(b) They have an electrophilic carbon and a good leaving group
(c) They have an electrophilic carbon and a bad leaving group
(d) They have a nucleophilic carbon and a good leaving group
ANSWER: (b) They have an electrophilic carbon and a bad leaving group
EXPLANATION: Alkyl halides have highly polar C—X bond in which X is highly partial negative while C is highly partial positive. The strong partial positive character of alpha carbon makes alkyl halides very favourable for nucleophilic attack. That’s why they are very reactive towards nucleophiles.
(ix) The rate of E1 reaction depends upon:
(a) The concentration of substrate
(b) The concentration of nucleophile
(c) The concentration of substrate as well as nucleophile
(d) None of the above
ANSWER: (a) The concentration of substrate
EXPLANATION: E1 or elimination unimolecular reactions complete in two steps. The first step is the ionization of alkyl halides in polar solvent to generate a carbocation. This is the slow step and thus rate determining step also. The only reactant in this step is the substrate molecule. Thus, the rate of E1 reaction depends solely upon the concentration of the substrate.
(x) Which one of the followings is not a nucleophile?
(a) H2O
(b) H2S
(c) BF3
(d) NH3
ANSWER: (c) BF3
EXPLANATION: BF3 is electron deficient species and has six electrons around the central B atom. It can, thus, accept a pair of electrons and act as an electrophile rather than a nucleophile.