Haloalkanes Flashcards
Reaction Map
General formula
CnH2n+1 X
Classification
- In a primary haloalkane the halogen is attached to a C, which is only attached to one other C.
- In a secondary haloalkane the halogen is attached to a C, which is only attached to two other C’s.
- In a tertiary haloalkane the halogen is attached to a C, which is attached to three other C’s.
FREE RADICAL definition
A free radical is a species which has a single unpaired electron
Why is UV light needed to start the reaction?
UV light provides the energy needed to break THE halogen covalent bond at the start of the reaction
Free radical substitution
Cl radical = catalyst
How can the amount of further substitution be reduced?
An excess of alkane must be used (which uses up all the halide radicals)
nucleophile definition
e- pair donor
base definition
accepts H+
Why are haloalkanes more reactive than alkanes
- haloalkanes contain polar bonds
- halogen has greater electronegativity than C atom
Nucleophilic Substitution by Hydroxide Ions (alcohol)
Reagent: NaOH (aq)
Conditions: Aqueous NaOH, Reflux (warm)
Equation: C2H5 Br + NaOH → C2H5OH + NaBr
Nucleophilic Substitution by Cyanide Ions (nitrile)
Nucleophilic Substitution with Ammonia (amine)
Conditions: Excess concentrated ammonia, ethanol solvent, heat and pressure.
C- Br has a difference in electronegativity
Cδ+ — Brδ-
lone pair on NH3 attracted to Cδ+
Elimination Reaction
Reagent: Concentrated NaOH / KOH(aq)
Conditions: Hot, ethanol solvent
(ethanolic conditions)
Equation: CH3CHBrCH3 + NaOH → CH2CHCH3 + NaBr + H2O
Hydrolysis definition
Splitting molecules using water
Method to see if hydrolysis has occurred
- Place a measured sample of each halogenoalkane, into separate test tubes and place in to a water bath at 50℃
- Into another test tube place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath.
- Once at equal temperature add equal volumes of the ethanol mixture and haloalkane solutions together.
- Time how long it takes for each precipitate to form.
The rate of this reaction can be determined by calculating 1/time taken for the precipitate to occur
Reason for the differing rates of hydrolysis
The stronger the bond the harder it is to break and the slower the rate of reaction will be as more energy is required to break it.
The weaker the bond the faster the rate will be.
C – I - weakest bond so fastest reaction, less energy require to break the bond
C – Cl - strongest bond so slowest reaction, more energy required to break the bond
The Ozone layer
Ozone is formed naturally in the upper atmosphere (stratosphere) by free radical reactions.
O2 is split into two by ultraviolet radiation into two oxygen atoms and then one of the oxygen atoms combines with an oxygen molecule to form ozone.
O2 → 2O▪
O2 + O ⇌ O3
Radicals can disrupt this balance, as they can catalyse the reaction to form oxygen
Uses of Haloalkanes
CFC’s were used extensively as they have useful properties
* non-toxic
* inert gases
* volatile
Uses
Refrigeration
Air conditioning
Dry cleaning solvents
How CFCs damage the Ozone layer
CFC’s are unreactive and so difficult to breakdown.
However once they reach the stratosphere they are broken down by UV
Alkanes, HFC’s hydrofluorocarbons and HCFC’s hydrochlorofluorocarbons are used frequently.
”ozone friendly” alternatives break down rapidly in the lower atmosphere, as they are more reactive in comparison to CFC’s, hence biodegradable.
