Haloalkanes and Haloarenes 5

Question 1

what are grignard reagents

Answer 1

An
important class of organo-metallic compounds discovered by Victor
Grignard in 1900 is alkyl magnesium halide, RMgX, referred as
Grignard Reagents. These reagents are obtained by the reaction of
haloalkanes with magnesium metal in dry ether.

CH3CH3Br +Mg —–dry ether—> CH3CH2MgBr

Question 1

what are organometallic cpds

Answer 1

Most organic chlorides, bromides and iodides react with certain
metals to give compounds containing carbon-metal bonds. Such
compounds are known as organo-metallic compounds.

Question 2

explain polarity and bond nature in grignard reagent

Answer 2

In the Grignard reagent, the carbon-magnesium bond is covalent
but highly polar, with carbon pulling electrons from electropositive
magnesium; the magnesium halogen bond is essentially ionic.

d- d+ d-
R–Mg–X

Question 3

why is grignard reagent prepared in
“dry ether”

Answer 3

Grignard reagents are highly reactive and react with any source of
proton to give hydrocarbons. Even water, alcohols, amines are sufficiently
acidic to convert them to corresponding hydrocarbons.

RMgX + H2O—-> R-H + Mg(OH)X

R-H - hydrocarbon
Mg(OH)X - magnesium hydroxyl halide

It is therefore necessary to avoid even traces of moisture from a Grignard
reagent. That is why reaction is carried out in dry ether. On the other
hand, this could be considered as one of the methods for converting
halides to hydrocarbons.

Question 4

what is wurtz reaction

Answer 4

Alkyl halides react with sodium in dry ether to give hydrocarbons
containing double the number of carbon atoms present in the halide.
This reaction is known as Wurtz reaction.

2RX +2Na —-dry ether—> RR + 2NaX

why dry ether - sodium reacts vigourously with mosture

Question 5

Aryl halides are extremely less reactive towards nucleophilic
substitution reactions. reason 1

Answer 5

Resonance effect : In haloarenes, the electron pairs on halogen
atom are in conjugation with p-electrons of the ring and the
following resonating structures are possible.

draw resonance structures for chlorobenzene

C—Cl bond acquires a partial double bond character due to
resonance. As a result, the bond cleavage in haloarene is difficult
than haloalkane and therefore, they are less reactive towards
nucleophilic substitution reaction.

Question 6

Aryl halides are extremely less reactive towards nucleophilic
substitution reactions. reason 2

Answer 6

Difference in hybridisation of carbon atom in C—X bond: In
haloalkane, the carbon atom attached to halogen is sp3
hybridised while in case of haloarene, the carbon atom attached
to halogen is sp2
-hybridised.

The sp2
hybridised carbon with a greater s-character is more
electronegative and can hold the electron pair of C—X bond
more tightly than sp3
-hybridised carbon in haloalkane with
less s-chararcter. Hence the Csp2-X bond is shorther Since it is difficult to
break a shorter bond than a longer bond, therefore, haloarenes
are less reactive than haloalkanes towards nucleophilic
substitution reaction.

Question 7

Aryl halides are extremely less reactive towards nucleophilic
substitution reactions. reason 3,4

Answer 7

(iii) Instability of phenyl cation: In case of haloarenes, the phenyl
cation formed as a result of self-ionisation will not be stabilised
by resonance and therefore, SN
1 mechanism is ruled out.
(iv) Because of the possible repulsion, it is less likely for the electron
rich nucleophile to approach electron rich arenes.

Question 8

Can you think why does NO2
group show its effect only at ortho- and para- positions
and not at meta- position?

Answer 8

As shown, the presence of nitro group at ortho- and para-positions withdraws the
electron density from the benzene ring and thus facilitates the attack of the nucleophile
on haloarene. The carbanion thus formed is stabilised through resonance. The negative
charge appeared at ortho- and para- positions with respect to the halogen substituent is
stabilised by –NO2 group while in case of meta-nitrobenzene, none of the resonating
structures bear the negative charge on carbon atom bearing the –NO2 group. Therefore,
the presence of nitro group at meta- position does not stabilise the negative charge and
no effect on reactivity is observed by the presence of –NO2
group at meta-position.

Question 9

the electrophilic substitution reactions
in haloarenes occur slowly and require more drastic conditions as
compared to those in benzene.

Answer 9

i) Haloarenes undergo the usual electrophilic reactions of the benzene
ring such as halogenation, nitration, sulphonation and Friedel-Crafts
reactions.

ii)Halogen atom besides being slightly deactivating is o, pdirecting; therefore, further substitution occurs at ortho- and parapositions with respect to the halogen atom.

iii)The o, p-directing influence
of halogen atom can be easily understood if we consider the resonating
structures of halobenzene:

iv)Due to resonance, the electron density increases more at ortho- and
para-positions than at meta-positions. Further, the halogen atom
because of its –I effect has some tendency to withdraw electrons from
the benzene ring.

v)As a result, the ring gets somewhat deactivated as
compared to benzene and hence the electrophilic substitution reactions
in haloarenes occur slowly and require more drastic conditions as
compared to those in benzene.