Chemistry Halogenoalkanes and reactions
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- All halogens contain at least one halogen (X) attached to a carbon atom
- If they contain just one halogen atom and there are no multiple bonds the general formula is CnH2n+1X
- The C-X bond is polarised which makes the carbon atom electron-deficent and open to attack by nucleophiles (e.g. water OH- ions from KOH)
- Becuase the halide ion is stable it is substituted easily by the nucleophile
Strength of the C-X bond. Strongest first
Reactivity of the halogenoalkanes. Most reactive first
Reactivity of the halogenoalkanes
- It depends on the breaking of the C-X bond
- As the C-F bond is the hardest to break the fluoroalkanes are the least reactive
Types of halogenoalkanes
- Primary - On the carbon with the halogen there are two hydrogen atoms
- Secondary - Pn the carbon with the halogen there is one hydrogen
- Tertiary - On the carbon with the halogen there are no hydrogen
Reactions of the halogenoalkanes
- The electon-deficient carbon in the carbon-halogen bond can be attacked by nucleophiles with their lone-pair electrons
- The carbon cannot form more than four bonds so the nucleophile forms a covalent bond with the carbon
- This causes the halogen to leave as X- (e.g. Br-) ion
Why are fluoroalkanes never used for synthesis of carbon compounds?
Synthesis relies on the halogen being replaced by another group and therefore it needs the C-F bond to break. This bond is very strong so it would take a long time.
Why are bromoalkanes used in syntheses in preference to iodoalkanes?
- The C-I bond is too weak so iodoalkanes often react with contaminents before than van be used in a reaction.
- Bromoalkanes are not so reactive that they are unstable or react too easily
What is a Nucleophile?
It is a molecule or ion that can donate a pair of electrons. It is often represented as Nu- or Nu:-
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