Hydrocarbons-4

Question 1

what helped to figure out the structure of benzene

Answer 1

i) Benzene adds 3H2 molecules to form cyclohexane
benzene+ 3H2—–> cyclohexane

ii) Benzene adds 3 molecules of chlorine( in presence of sunlight/ absance of hydrogen carrier) to form benzene hexachloride

iii) undegoes ozonolysis to form triozonide molecule, indicating 3 double bonds are present

iv)Benzene was further found to produce one and only one
monosubstituted derivative which indicated
that all the six carbon and six hydrogen atoms
of benzene are identical.

Question 2

benzene acts like a saturated compound. justify

Answer 2

• it udnergoes substitution reactions( electrophilic substititution reactions) rather than addition reactions( unlike alkene,alkyne). Resonance already stabilises the molecule.

-benzene resists oxidation unlike alkynes and alkenes. it is only slowly oxidised to carbon dioxide and water. Benzene resists oxidation just like saturated compounds

Question 3

explain the structure of benzene, and hence is stability

Answer 3

• Benzene consists of 6 sp2 hybridised orbitals. Each carbon has an unhybridised p orbital.
• there is equal probability for the p
  orbital of each carbon atom to overlap with the
  p orbitals of adjacent carbon atoms.
  -Hence the pi electron clouds lie above and below the plane of the benzene rings, in the form of rings.
  -The six π electrons are thus delocalised and
  can move freely about the six carbon nuclei,
  instead of any two as shown in Fig. 13.6 (a) or
  (b). The delocalised π electron cloud is attracted
  more strongly by the nuclei of the carbon
  atoms than the electron cloud localised
  between two carbon atoms.

the delocalised electrons are equally attracted by all 6 carbons of benzene

Question 4

what are the conditions for a compound to be aromatic

Answer 4

(i) It should have a single cyclic electron cloud above and below the plant of the molecule(Complete delocalisation of the π electrons
in the ring
)
ii) the molecule should be planar( only then will complete delocalisation of electrons be possible)
iii) there must be 4n+2 pi electrons

these are called huckel rule

Question 5

explain the methods of preparation of benzene

Answer 5

Benzene is commercially isolated from coal tar

(i) Cyclic polymerisation of ethyne:
(Section 13.4.4)
(ii) Decarboxylation of aromatic acids:
Sodium salt of benzoic acid on heating with
sodalime gives benzene.
cao acts as inert supportn
iii) Reduction of phenol: Phenol is reduced
to benzene by passing its vapours over
heated zinc dust
iv) Aromatisation or reforming of n-alkanes

Question 6

what are the physical properties of arenes

Answer 6

Aromatic hydrocarbons are non- polar
molecules and are usually colourless liquids
or solids with a characteristic aroma. You are
also familiar with naphthalene balls which are
used in toilets and for preservation of clothes
because of unique smell of the compound and
the moth repellent property. Aromatic
hydrocarbons are immiscible with water but
are readily miscible with organic solvents. They
burn with sooty flame.

Question 7

what are the most common electrophilic substitution reactions

Answer 7

(i)Nitration: A nitro group is introduced into
benzene ring when benzene is heated with a
mixture of concentrated nitric acid and
concentrated sulphuric acid (nitrating
mixture).

benzene—-conc. hno3+ conc h2so4 / 323-333K —> nitrobenzene+ water

(ii) Halogenation: Arenes react with halogens
in the presence of a Lewis acid like anhydrous
FeCl3, FeBr3 or AlCl3 to yield haloarenes. Lewis acid acts as halogen carrier and electron rich halogen cannot directly attack on electron rich benzene.

iii)(iii) Sulphonation: The replacement of a
hydrogen atom by a sulphonic acid group in
a ring is called sulphonation. It is carried out
by heating benzene with fuming sulphuric acid
(oleum).
oleum= h2so4(so3)

Question 8

explain the friedal crafts reactions

Answer 8

(iv) Friedel-Crafts alkylation reaction:
When benzene is treated with an alkyl halide
in the presence of anhydrous aluminium
chloride, alkylbenene is formed.

Friedel-Crafts acylation reaction: The
reaction of benzene with an acyl halide or acid
anhydride in the presence of Lewis acids (AlCl3
)
yields acyl benzene.

Question 9

what is mechanism of electrophilic substitution reactions

Answer 9

According to experimental evidences, SE
(S =
substitution; E = electrophilic) reactions are
supposed to proceed via the following three
steps:
(a) Generation of the eletrophile
(b) Formation of carbocation intermediate
(c) Removal of proton from the carbocation
intermediate

Question 10

i) generation of electrophile

Answer 10

During
chlorination, alkylation and acylation of
benzene, anhydrous AlCl3
, being a Lewis acid
helps in generation of the elctrophile Cl⊕
, R⊕
,
RC⊕O (acylium ion) respectively by combining
with the attacking reagen

In the case of nitration, the electrophile,
nitronium ion, NO2
+
is produced by transfer
of a proton (from sulphuric acid) to nitric acid
in the following manner. In this reaction sulphuric acts as acid and nitric acid acts a base

Question 11

ii) formation of carbocatiin( aernium ion/ sigma complex)

Answer 11

(b) Formation of Carbocation
(arenium ion): Attack of electrophile
results in the formation of σ-complex or
arenium ion in which one of the carbon is sp
3
hybridised. This carbon is attached to the eelctrophile.

The arenium ion gets stabilised by
resonance.Sigma complex or arenium ion loses its
aromatic character because delocalisation of
electrons stops at sp
3
hybridised carbon

Question 12

iii) removal of proton from carbocation

Answer 12

To restore the
aromatic character, σ -complex releases proton
from sp
3
hybridised carbon on attack by
[AlCl4
]
–
(in case of halogenation, alkylation and
acylation) and [HSO4
]
–
(in case of nitration).

Question 13

can there be excessive substitution? explain

Answer 13

If excess of electrophilic reagent is used,
further substitution reaction may take place
in which other hydrogen atoms of benzene ring
may also be successively replaced by the
electrophile. For example, benzene on
treatment with excess of chlorine in the
presence of anhydrous AlCl3
can be
chlorinated to hexachlorobenzene (C6Cl6
)

Question 14

explain addition reactions shown by benzene

Answer 14

Under vigorous conditions, i.e., at high
temperature and/ or pressure in the presence
of nickel catalyst, hydrogenation of benzene
gives cyclohexane.

Under ultra-violet light, three chlorine
molecules add to benzene to produce benzene
hexachloride, C6H6Cl6
which is also called
gammaxane/lindane. pesticide

Question 15

why doesnt benzene show addition reactions

Answer 15

it is due to stability due to resonance. In benzene, the pi electron are present on each unhybridised 2p orbitals of carbon and makes the structure more stable.

Question 16

what is directive influence?

Answer 16

When monosubstituted benzene is subjected to furher substitution, the position of the addition of substituent is determined by the nautre of the substituent already present on the ring( not by the substitutent entering the ring). This is called as direcitve influence.

Question 17

what are orthopara directing groups

Answer 17

The
groups which direct the incoming group to
ortho and para positions are called ortho and
para directing groups.

the electron density is more on
o – and p – positions. Hence, the substitution
takes place mainly at these positions.

These groups increase the overall electron density in the benzene ring. Hence they activate the benzene ring for attack by an electrophile and called activating groups.e –NH2
, –NHR, –NHCOCH3
, –OCH3
,
–CH3
, –C2H5
, etc.

Question 18

are halogens o/p rdirecting or m directing

Answer 18

In the case of aryl halides, halogens are
moderately deactivating. Because of their
strong – I effect, overall electron density on
benzene ring decreases. It makes further
substitution difficult. However, due to
resonance the electron density on o– and p–
positions is greater than that at the m-position.
Hence, they are also o– and p– directing groups

Question 19

describe meta directing groups

Answer 19

The groups which
direct the incoming group to meta position are
called meta directing groups. Some examples
of meta directing groups are –NO2
, –CN, –CHO,
–COR, –COOH, –COOR, –SO3H, etc.

In this case, the overall electron density on
benzene ring decreases making further
substitution difficult, therefore these groups
are also called ‘deactivating groups’. The
electron density on o– and p– position is
comparatively less than that at meta position.
Hence, the electrophile attacks on
comparatively electron rich meta position
resulting in meta substitution.

Question 20

explain carcinogenecity and toxicity

Answer 20

Benzene and polynuclear hydrocarbons
containing more than two benzene rings
fused together are toxic and said to possess
cancer producing (carcinogenic) property.
Such polynuclear hydrocarbons are formed
on incomplete combustion of organic
materials like tobacco, coal and petroleum.
They enter into human body and undergo
various biochemical reactions and finally
damage DNA and cause cancer.