Haloalkanes and Haloarenes Flashcards
state the uses of the following haloalkanes/haloarenes:
(i) Chloramphenicol
(ii)Thyroxine
(iii)Chloroquine
(iv)Halothane
(v)Per-fluorinated compounds
They are used as solvents for relatively
non-polar compounds and as starting materials for
the synthesis of wide range of organic compounds.
(i) Chlorine containing compound produced by soil microorganisms which is effective in treating typhoid fever
(ii) Iodine containing hormone produced in our body deficiency of which causes goitre. ( abnormal swelling of neck).
(iii) Synthetic halogen compound which is used in the treatment of malaria
(iv) It is used as a dental anaesthetic in surgery( CF3-CHClBr)
(v) Fully fluorinated compounds that are potential blood substitutes during long surgeries
classification of halogen compounds based on number of halogen atoms attached
These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.)
compounds depending on whether they contain one, two or more halogen
atoms in their structures.
classification of halogen ompounds on the basis of hybridistion
sp3 hybridised
I) sp3 hybridised
i) Alkyl halides
In alkyl halides, the halogen atom is bonded to an alkyl group (R).
They form a homologous series represented by CnH2n+1X. They are
further classified as primary, secondary or tertiary according to the
nature of carbon to which halogen is attached. If halogen is attached to
a primary carbon atom in an alkyl halide, the alkyl halide is called
primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached
to secondary or tertiary carbon atom, the alkyl halide is called
secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively.
ii)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.
iii) Benzylic halides
These are the compounds in which the halogen atom is bonded to an
sp3
-hybridised carbon atom attached to an aromatic ring.
classification of halogen ompounds on the basis of hybridistion
sp2 hybridised
(a) 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
(C = C).
(b) Aryl halides
These are the compounds in which the halogen atom is directly
bonded to the sp2
-hybridised carbon atom of an aromatic ring.
describe the nature of c-x bond
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.
explain the variation of bond length and bond enthalpy of c-x bond
As we go down the group in the periodic table, the size of halogen
atom increases. Fluorine atom is the smallest and iodine atom is the
largest. Consequently the carbon-halogen bond length also increases
from C—F to C—I.
Bond length is inversely proportional to bond enthalpy. Longer the bond, lesser is the energy required to cleave it. So bond enthalpy decreases down the group from C—F to C—I
why is the dipole moment of C-F bond lesser thn C-Cl bond even though F is more electronegative?
The dipole moment of the CH3-F is lesser than CH3-Cl. The magnitude of -ve charge on the F atom is greater, however the C-F bond has a smaller bond legnth due to the smaller size of fluorine atom.
Dipole moment = charge x distance of seperation
since as bond legnth decreases, distance of seperation of charges decreases, dipole moment of CH3F decreases.
why is using thionyl chloride the best way to prepare haloalkanes
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.
R-OH + SOCl2 ——–> R-Cl + SO2↑ + HCl↑
how to form alkyl chloride from alcohol using hcl/hbr
The reactions of primary and
secondary alcohols with HCl require the presence of a catalyst, ZnCl2
.
With tertiary alcohols, the reaction is conducted by simply shaking the
alcohol with concentrated HCl at room temperature.
R-OH + HCl ——> R-Cl + H2O
Bromoalkanes can be prepared by refluxing a suitable alcohol with constant boiling hbr(48%)
R-OH + NaBr + H2SO4 ——> R-Br + NaHSO4 + H2O
Iodoalkanes can be prepare dby refluxing alcohol with constant boiling hi.
ROH + HI —–> RI + H2O
what is the function of ZnCl2 catalyst
ZnCl2 helps in breaking of the carbnon-oxygen bonds. It is a lewis acid and coordinates with the oxygen atom of the alcohols, hence the carbon-oxygen bond weakens and breaks to form the carbocation.
how to produce alkyl iodide
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°.
R-OH + NaI/KI + H3PO4 ——> R-I + NaH2PO4 + H2O
why is h2so4 not used with k/nai for making iodoalkanes
Sulphuric acid is a strong oxidising agent. There h2so4 will oxidise the produced HI to I2 and hence the reaction will not proceed.
how can we use red phosphorous to prepare alkyl halide
. Phosphorus tribromide and triiodide
are usually generated in situ (produced in the reaction mixture) by the
reaction of red phosphorus with bromine and iodine respectively.
R-OH —red P/ Br2/I2——> R-Br/ R-I
how can we prepare alkyl halide using phosphorous halides
3R-OH + PX3 ——> 3R-X + H3PO3 ( X= cl/br)
R-OH + PCl5 ——-> R- Cl + POCl3 + HCl
why are certain methods of preparation not suitable for aryl halides
The preparation of alkyl chloride is carried out 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 above methods are not applicable for the preparation of aryl
halides because the carbon-oxygen bond in phenols has a partial double
bond character and is difficult to break being stronger than a single
bond.
