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AQA A-Level Chemistry - Halogenoalkanes Summary
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A summary of the Halogenoalkanes topic from the AQA A-Level Chemistry Course.
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O - HalogenoalkanesHaloalkanes are organic compounds in which there is one (or more) halogen(s)
attached to the hydrocarbon chain (as a result of one (or more) hydrogen atom(s)
being replaced by halogen atom(s)).
They are not commonly found in nature but are the basis of many synthetic
compounds (like PVC, Teflon).
The general formula for haloalkanes with one halogen atom is CnH2n+1X where X is
the halogen atom
The IUPAC system can be applied to haloalkanes. The prefixes fluoro-, chloro-,
bromo- and iodo- are added to indicate the type of halogen atom. Di-, tri-, tetra- are
used to indicate number of each of the halogen atoms, and numbers are used to
indicate position of the halogen in the chain. When arranging the name of
halogenoalkane with multiple halogens, follow alphabetical order.
Halogenoalkanes are more reactive than alkanes due to the polar C-X bond. The C-
X bond is polar because the halogen atoms are more electronegative than the
carbon atom resulting in the X atom to pull the electron density towards it, resulting
in a permanent dipole - which is the polar bond.
Bond polarity decreases from C-F to C-I
Due to the carbon atom being delta positive, it attracts electron-rich nucleophiles
towards it.
The bond enthalpies of C-X bonds decreases from C-F to C-I:
The C-F bond has the highest bond enthalpy as the bond is the shortest, hence
the bond is hardest to break, hence the C-F bond is the least reactive.
The C-I bond has the lowest bond enthalpy as the bond is the longest, hence the
bond is easiest to break, hence the C-I bond is most reactive.
The bond polarity of C-X bonds decreases from C-F to C-I:
C-F is the most electronegative, hence the most polar because shared paired of
electrons are closest to the halogen. Hence, C-F bonds have the highest
tendency for a nucleophilic attack
O - Halogenoalkanes 1
, C-I is the least electronegative, hence, the least polar because shared paired of
electrons are furthest away from the halogen. This leads to the C-I bond having
the lowest tendency for nucleophilic attacks.
From C-F to C-I:
1. Bond enthalpy decreases → Because increasing bond length → Weaker bonds
→ Becomes easier to break bond → More reactive
2. Bond polarity decreases → Because electronegativity of X decreases → Lower
tendency for nucleophilic attacks as C atom becomes less delta positive
Conclusion: The bond enthalpy is a more important factor than bond polarity
when determining reactivity. (Overall: iodoalkanes are most reactive).
Physical properties:
1. Solubility:
a. The polar C-X bonds are not polar enough to make the haloalkanes soluble
in water. Also, the parent chain only has carbon atoms bonded to hydrogen
atoms. The C-H is not polar enough to form hydrogen bonds with water,
hence, haloalkanes are insoluble in water. Overall, haloalkanes cannot form
hydrogen bonds, so they are insoluble in water.
b. The main intermolecular forces are temporary dipole-dipole interactions (Van
Der Waals’ forces).
c. Haloalkanes mix with hydrocarbons so they can be used as dry-cleaning
fluids and to remove oily stains (as oil is a mixture of hydrocarbons)
2. Boiling point trends:
a. Boiling point of haloalkanes is higher than that of alkanes with the same
number of carbon atoms because of the increased molecular mass, more
polar bonds, and larger surface area resulting in the Van Der Waals’ forces
to be more effective.
b. Boiling point depends on the number of carbon atoms and the type and
number of halogen in the parent molecule
c. Boiling point increases as the number of carbon atoms in the parent chain
increases
d. Boiling point increases going down the halogen group
O - Halogenoalkanes 2
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