haloalkanes Flashcards
explain the formation of alkyl halide and aryl halide
The replacement of hydrogen atom(s) in an aliphatic or aromatic hydrocarbon by halogen atom(s) results in the formation of alkyl halide and aryl halide
what is the diff between Haloalkanes and haloarenes
Haloalkanes contain halogen atom(s) attached to the sp3 hybridized carbon
atom of an alkyl group whereas haloarenes contain halogen atom(s) attached to sp2
hybridized carbon atom(s) of an aryl group
what is chloramphenicol used for?
Chlorine containing antibiotic produced by microorganisms is very effective for the
treatment of typhoid fever
the deficiency of what can cause thyroxine
Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease called goiter
chloroquine is used for the treatment
of
malaria
what is the classification of Haloalkanes and haloarenes Containing sp3 Carbon
Alkyl halides
Allylic halides
Benzylic halides
explain alkyl halides
the halogen atom is bonded to an alkyl group (R). They form a homologous series represented by CnH2n+1X
explain allylic halides
These are the compounds in which the halogen atom is bonded to an
sp3-hybridised carbon atom adjacent to carbon-carbon double bond
(C=C) i.e. to an allylic carbon
explain Benzylic halides
These are the compounds in which the halogen atom is bonded to an
sp3-hybridised carbon atom attached to an aromatic ring.
what is the classification of Haloalkanes and haloarenes Compounds Containing sp2 C
Vinylic halides
Aryl halides
what is vinylic halides?
These are the compounds in which the halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-carbon double bond
what is Aryl halides?
These are the compounds in which the halogen atom is directly bonded to the sp2-hybridised carbon atom of an aromatic ring.
what is geminal halides?
when both the halogen atoms are present on the same carbon atom of the chain
what is vicinal halides
when halogen atoms are present
on adjacent carbon atoms
Nature of C-X Bond (polarised or not polarised)
Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised the carbon atom bears a partial positive charge whereas the halogen atom bears a partial
negative charge
As we go down the group in the periodic table, the size of halogen
atom
increases
_____atom is the smallest halogen and _______ atom is the largest
Fluorine and iodine
Alkyl halides are best prepared from
alcohols
Methods of Preparation of Haloalkanes from alcohols
The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids like phosphorus halides or thionyl chloride
why is thionyl chloride preferred more in Preparation of Haloalkanes from alcohols
Thionyl chloride is preferred because in this reaction alkyl halide is formed along with gases SO2 and HCl. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides
. The reactions of primary and
secondary alcohols with HCl require the presence of a catalyst, ______
ZnCl2
Methods of Preparation of Haloalkanes with tertiary alcohols
the reaction is conducted by simply shaking the alcohol with concentrated HCl at room temperature Constant boiling
with HBr (48%) is used for preparing alkyl bromide
Good yields of R—I may be obtained by
heating alcohols with sodium or potassium
iodide in 95% orthophosphoric acid
The order of reactivity of alcohols
with a given haloacid is
3°>2°>1°
The preparation of alkyl chloride is carried out by?
either by passing dry hydrogen chloride gas through a solution of alcohol or by heating
a mixture of alcohol and concentrated aqueous halogen acid.
the methods of preparation of alcohol is not used to prep aryl halides because
because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond
Methods of Haloalkanes Preparation From Hydrocarbons
From alkanes by free radical halogenation
From alkenes ((i) Addition of hydrogen halides:, (ii) Addition of halogens)
Preparation of haloalkanes from alkanes by free radical halogenation
Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds. Consequently, the yield of any single
compound is low
Preparation of haloalkanes from alkenes
(i) Addition of hydrogen halides; An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide Propene yields two products, however only one predominates as per Markovnikov’s rule
(ii) Addition of halogens ;addition of bromine inCCl4 to an alkene resulting in discharge of reddish brown colour
of bromine constitutes an important method for the detection of
double bond in a molecule The addition results in the synthesis of vic-dibromides, which are colourless
Preparation of haloalkanes from Halogen Exchange
(i)Alkyl iodides are often prepared by the reaction of alkyl chlorides/bromides with NaI in dry acetone. This reaction is known as Finkelstein reaction
(ii)The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg2
F2 The reaction is termed as Swarts reaction
Finkelstein reaction
R-X + NaI = R-I + NaX
Swarts reaction
CH3-Br + AgF = CH3-F + AgBr
method of Preparation of Haloarenes
(i) From hydrocarbons by electrophilic substitution
(ii) From amines by Sandmeyer’s reaction
Preparation of Haloarenes From hydrocarbons by electrophilic substitution
Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride.
problems caused during Preparation of Haloarenes From hydrocarbons by electrophilic substitution
The ortho and para isomers can be easily separated due to large difference in their melting points.
Reactions with iodine are reversible in nature and require the presence of an oxidising agent (HNO3,HIO4) to oxidise the HI formed during iodination. Fluoro compounds are not prepared by this method due to high reactivity of fluorine
Preparation of Haloarenes From amines by Sandmeyer’s reaction
When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium
salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the
replacement of the diazonium group by –Cl or –Br
Alkyl halides are _______ when pure
colourless However, bromides and iodides
develop colour when exposed to light.
why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass
Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals)
are stronger in the halogen derivatives
The boiling points of isomeric haloalkanes decrease with ______
in branching
increase
why the para-isomers are high melting as compared to their ortho and meta-isomers
It is due to symmetry of para-isomers that fits in crystal lattice better as compared to ortho- and meta-isomers
the solubility of haloalkanes in water is low why
In order to dissolve haloalkane in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between water molecules. Less energy is released when new attractions are set up between the haloalkane and the water molecules as these are not as strong as the
original hydrogen bonds in water
haloalkanes tend to dissolve in organic solvents why
the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules.
what are nucleophiles?
nucleophiles are electron rich species.
Therefore, they attack at that part of the substrate molecule which
is electron deficient
what is nucleophilic substitution reaction
The reaction in which a nucleophile replaces already existing nucleophile in a molecule is called nucleophilic substitution reaction
what are ambident nucleophiles
Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles.
how does nucleophilic substitution reaction take place
These two processes take place simultaneously in a single step and no intermediate is formed. As the reaction progresses and the bond between the incoming nucleophile and the carbon
atom starts forming, the bond between carbon atom and leaving
group weakens As this happens, the three carbon-hydrogen bonds of the substrate start moving away from the attacking nucleophile. In transition state all the three C-H bonds are in the same plane and the
attacking and leaving nucleophiles are partially attached to the carbon. As the attacking nucleophile approaches closer to the carbon, C-H bonds still keep on moving in the same direction till the attacking
nucleophile attaches to carbon and leaving group leaves the carbon. As a result configuration is inverted, This
process is called as inversion of configuration.
