Haloalkanes Flashcards
what are haloalkanes?
Haloalkanes are alkanes (saturated hydrocarbons) which have had one or
more hydrogen atoms substituted with a halogen atom
what is the general formula of haloalkanes?
CnH2n+1X
X is the halogen
how are haloalkanes named?
Haloalkanes are named by adding the prefix halo (i.e. fluoro, chloro, bromo,
iodo) to the beginning of the alkane name
how do you classify the haloalkane?
- Identify the number of alkyl groups/carbons attached to the carbon bonded to the halogen –
if only 1 is present then the haloalkane is primary, 1o,
if 2 then secondary, 2o,
if 3 then tertiary, 3o
why are alkanes unreactive?
+ C-C & C-H bonds are strong
+ Non-polar
why are haloalkanes reactive?
+ C-X bond polar
+ C-X bond weaker
what is the difference between the reactivity of halo and alkanes?
The halogen (X) is more electronegative than carbon
what does the electronegativity mean?
The ability of an atom to attract the electrons to itself in a covalent bond.
what two types of reactions do haloalkanes undergo?
- nucleophilic substitution
- elimination reactions.
what is a nucleophile?
a lone pair/ electron pair donor
nucleophilic substitution - what is the electron pair attracted to?
The electron pair is attracted to the δ+ carbon of the haloalkane.
nucleophilic substitution - what happens to the carbon-halogen
The carbon-halogen bond is forced to break and the new compound is formed.
why is the nucleophillic substitution called this?
The reaction is substitution because the halogen is replaced by the nucleophile.
what is the Reagent used in hydrolysis?
sodium hydroxide solution (NaOH (aq))
what are the conditions for hydrolysis?
aqueous solution, warm
what is the general equation fir hydrolysis?
R-X +NaOH (aq) –> R-OH +NaX
Where R is any alkyl group and X is any halogen
How do primary haloalkanes react?
Primary haloalkanes react by an SN2 mechanism (this means that two species are involved in the slowest step of the nucleophilic substitution mechanism).
how do tertiary haloalkanes react?
Tertiary haloalkanes react by an SN1 mechanism (this means that only one species is involved in the slowest step of the nucleophilic substitution mechanism).
How do secondary haloalkanes react?
Secondary haloalkanes can react by either mechanism
(you can use either mechanism if asked to describe the mechanism for a 2o haloalkane.)
sn2 mechanism with 1o haloalkanes
2 species involved in slowest step SN2. haloalkane & OH-
Simultaneous bond forming & bond breaking.
The SN2 Mechanism with 1° Haloalkanes - key marking points?
- Dipole: on C – X bond
- Curly arrows: must start
between lone pair or on a
bond - Lone pairs: shown on
correct atom of nucleophile - Charges: on nucleophile and
the halide ion produced
what happens in the SN2 mechanism?
In the SN2 mechanism, the nucleophile (OH-) attacks the haloalkane in the first, slow step,
of the mechanism, which forces the carbon-halogen bond to break in the same step.
SN1 Mechanism with 3o Haloalkanes
Only 1 species involved in slowest step SN1
C-Br bond breaking in haloalkane
what happens in the SN1 mechanism?
In the SN1 mechanism, the haloalkane first breaks down in the initial slow step (so only one species is involved), forming a relatively stable tertiary carbocation, which then attracts the nucleophile (OH-) to produce the product.
