Hydrocarbons -2 Flashcards
what are geometrical isomerS?
restricted rotation of atoms or groups around
the doubly bonded carbon atoms gives rise to
different geometries of compounds. such isomers are called geometrical isomers
what are cis and trans isomers
The isomer of the type
(a), in which two identical atoms or groups lie
on the same side of the double bond is called
cis isomer and the other isomer of the type
(b), in which identical atoms or groups lie on
the opposite sides of the double bond is called
trans isomer . Thus cis and trans isomers
have the same structure but have different
configuration (arrangement of atoms or groups
in space).
which geometrical isomers has:
i) higher mpt
ii) higher bpt
i) The trans isomer has a higher melting point because due its symmetrical structure, it can pack more well in a crystal lattice thereby increasing the force of attraction between molecules.
ii)the cis isomer is more polar than the trans form. The dipoles of the c-ch3 bond in trans isomers cancel each other, as they are direction in opposite direction, making the trans molecule non polar.
therefore the cis isomer will have higher boiling point due to stronger intermolecular force of attraction ( dipole-dipole atrtracttion forces)
how do you convert an alkyne to an alkene
Alkynes on partial
reduction with calculated amount of
dihydrogen in the presence of palladised
charcoal partially deactivated with poisons
like sulphur compounds or quinoline give
alkenes. Partially deactivated palladised
charcoal is known as Lindlar’s catalyst.
Alkenes thus obtained are having cis
geometry. However, alkynes on reduction
with sodium in liquid ammonia form trans
alkenes.
how can an alkene be obtained fromalkyl halide
Alkyl halides (R-X)
on heating with alcoholic potash
(potassium hydroxide dissolved in alcohol, eliminate one molecule of
halogen acid to form alkenes. This reaction
is known as dehydrohalogenation i.e.,
removal of halogen acid. This is example of
β-elimination reaction, since hydrogen
atom is eliminated from the β carbon atom
(carbon atom next to the carbon to which
halogen is attached).
Nature of halogen atom and the alkyl
group determine rate of the reaction. It is
observed that for halogens, the rate is:
iodine > bromine > chlorine, while for alkyl
groups it is : tert > secondary > primary.
how do you convert a vicinal dihalide to an alkene
Vicinal dihalides on
treatment with zinc metal lose a molecule
of ZnX2
to form an alkene. This reaction is
known as dehalogenation.
how to convert alcohol to alkene
Alcohols on heating
with concentrated sulphuric acid form
alkenes with the elimination of one water
molecule. Since a water molecule is
eliminated from the alcohol molecule in the
presence of an acid, this reaction is known
as acidic dehydration of alcohols
describe phy properties of alkenes
The first three
members are gases, the next fourteen are
liquids and the higher ones are solids. Ethene
is a colourless gas with a faint sweet smell. All
other alkenes are colourless and odourless,
insoluble in water but fairly soluble in nonpolar solvents like benzene, petroleum ether.
They show a regular increase in boiling point
with increase in size i.e., every – CH2
group
added increases boiling point by 20–30 K. Like
alkanes, straight chain alkenes have higher
boiling point than isomeric branched chain
compounds.
why are alkenes easily attacked by electrophilic reagents
the presence of the pi (π) bond makes alkenes
behave as sources of loosely held mobile
electrons. Therefore, alkenes are easily attacked
by reagents or compounds which are in search
of electrons. Such reagents are called
electrophilic reagents. The presence of
weaker π-bond makes alkenes unstable
molecules in comparison to alkanes and thus,
alkenes can be changed into single bond
compounds by combining with the
electrophilic reagents.
explain the addition of halogens to alkenes
Halogens like
bromine or chlorine add up to alkene to
form vicinal dihalides. However, iodine
does not show addition reaction under normal conditions. The reddish orange
colour of bromine solution in carbon
tetrachloride is discharged when bromine
adds up to an unsaturation site. This
reaction is used as a test for unsaturation.
Addition of halogens to alkenes is an
example of electrophilic addition reaction
involving cyclic halonium ion formation
explain the addition of hydrogen halides
Hydrogen halides (HCl, HBr,HI) add up to
alkenes to form alkyl halides. The order of
reactivity of the hydrogen halides is
HI > HBr > HCl. Like addition of halogens
to alkenes, addition of hydrogen halides is
also an example of electrophilic addition
reaction.
state markovnikov’s rule
Markovnikov’s rule states that,” When an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part of the addendum gets added to the carbon having the lesser no of hydrogen atoms”
explain the mechanims behind markovnivkov’s rule
i)Hydrogen bromide provides an electrophile, H+
HBr—–> H+ + Br-
ii) The electrophile H+ attacks the double bond to form carbocation
write the structure of 2 carbocations
(i) The secondary carbocation (b) is more
stable than the primary carbocation (a),
therefore, the former predominates because
it is formed at a faster rate.
The primary carbocation is less stable and hence is formed ata much slower rate.
(iii) The carbocation (b) is attacked by Br–
ion
to form the product
2 products are formed ( mark major and minor prdt)
what is antimarkovnikov/ kharash effect/ peroxide effect
In the presence of peroxide, addition of HBr
to unsymmetrical alkenes like propene takes
place contrary to the Markovnikov rule. This
happens only with HBr but not with HCl and
Hl. This addition reaction was observed
by M.S. Kharash and F.R. Mayo in 1933 at
the University of Chicago. This reaction
is known as peroxide or Kharash effect
or addition reaction anti to Markovnikov
rule.
In the presence of a peroxide, when an unsymmetrical reagent is added to an unsymmetrical alkene, the negative part of the addendum is added to the carbon having more number of hydrogen atoms.
: Peroxide effect proceeds via free
radical chain mechanism
explain the addition of sulphuric acid to alkene
Cold
concentrated sulphuric acid adds to
alkenes in accordance with Markovnikov
rule to form alkyl hydrogen sulphate by
the electrophilic addition reaction.
