Halogenoalkanes Flashcards
General formula for halogenoalkanes
CnH2n+1X (where X is the halogen such as Br or Cl or I)
Primary RX
When the carbon atom bonded to the halogen atom is attached to one other carbon atom
Secondary RX
When the carbon atom bonded to the halogen atom is attached to two other carbon atoms
Tertiary RX
When the carbon atom bonded to the halogen atom is attached to three other carbon atoms
What is the reactions of halogenoalkanes called and what happens
Nucleophilic substitution reactions, where the halogen atom is replaced by the nucleophile
why is the C-X bond polar
Due to the difference in electronegativity. The halogen atom is more electronegative than the carbon atom
What attacks the electron deficient carbon atom
Nucleophiles such as OH-, CN-, NH3 and H20
What is a nucleophile
A species that donates a lone pair of electrons. A nucleophile is attracted to an electron deficient centre
What happens in the hydrolysis of halogenoalkanes by aqeuous alkali
When a halogenoalkane is heated under reflux with AQ NaOH or KOH, the halogen atom is replaced by the nucleophile, OH-, and an alcohol is produced
What happens in the hydrolysis of halogenoalkanes by water
Water is a poor nucleophile but it can slowly displace halide ions from a halogenoalkane
What does hydrolysis mean
the C-X bond is broken using water
What happens in the reaction between halogenoalkanes with alcoholic solution of potasisium cyanide (CN)
When a halogenoalkane is heated under reflux with KCN dissolved in ethanol, the halogen atom is replaced by the nucleophile CN-, and a nitrile is formed
what is the condition needed for nucleophillic substituition
Heat under reflux
What organic product is formed by nucleophilic substitution
Alcohol
What are halogenoalkanes dissolved in
ethanol since they are insoluble in water
What colour precipitate does AgCl form
White
What colour precipitate does AgBr form
Cream
What colour precipitate does AgI form
Yellow
What is the condition needed for nucleophilic substitution of ammonia dissolved in ethanol
heat in a sealed tube
What is ammonia used as in the nucleophilic substitution reaction
in the first step - as a nucleophile as it donates a lone pair of electrons
in the second step - as a base as it reacts with a H+ ion to form an NH4 + ion
Describe the general test for a halogenoalkane
Add aqeuous sodium hydroxide and heat.
Then add excess dilute nitric acid followed by silver nitrate solution and note the colour of the precipitate formed.
Then add dilute aqeuous ammonia or concentrated aq ammonia to the precipitate and note the solubility of the precipitate
When do you use dilute aq ammonia in the test for halogenoalkanes and why
To differentiate between white + cream precipitate or white + yellow precipitate as Cl is solube in dilute aq ammonia and Br is slightly soluble and I is insoluble
When do you use concentrated aq ammonia in the test for halogenoalkanes and why
To differentiate between cream and yellow precipitates because Br is soluble in this but I is not soluble
Why is sodium hydroxide solution added when testing for halogenoalkanes
To hydrolyse the halogenoalkanes and liberate the halide ion
Why is dilute nitric acid added before the silver nitrate solution in the test for halogenoalknes
Nitric acid is added to neutralise the sodium hydroxide. The OH- ions would interfere with the test because they react with Ag+ to form a precipitate
What is the trend in reactivity of halogenoalkanes
iodine > bromine > chlorine
What bonds do halogenoalkanes contain
Polar bonds
Why are iodoalkanes the most reactive
The bond enthalpy of C-I < C-Br < C-CI, so the C-I bond is the weakest.
Less energy is needed to break the C-I bond, so iodoalkanes react faster than the others
What happens to the C-X bond as you go down group 7
The bond gets larger and therefore weaker as you go down G7
What happens in elimination reactions
X leaves as X-
OH- removes a H+ ion from the C atom next to the C with the halogen
An alkene is formed
What is the condition needed for an elimination reaction
Heat
How to tell if the reaction is elimination or nucleophilic substitution
Elimination reaction uses a reagent dissolved in ethanol
Nucleophilic substitution uses an aq reagent
Why are a mixture of alkenes formed when a secondary halogenoalkane reacts with KOH dissolved in ethanol
The H+ ion can be removed from a carbon atom on either side of the carbon atom that is bonded to the halogen atom
What does OH- act as when a HX reacts with KOH dissolved in ethanol
a base as it removes a H+ ion
What kind of compounds can elimination reactions never form
Branched compounds
why were CFC’s used as refridgerants
they are unreactive ,not toxic and not corrosive
why are CFC’S no longer used
They are the main source of chlorine free radicals which react with ozone and break down the Earth’s protective ozone layer
Where is ozone formed
naturally in the upper atmosphere
Why is ozone beneficial
It absorbs ultraviolet radiation
What are the dangers of ultraviolet radiation entering the Earth’s atmosphere
causes skin cancer and eye cataracts
Why are HFC’s used instead of CFC’s now
HFC’s are not broken down by ultraviolet radiation as the C-F bonds are very strong and no chlorine free radicals can be formed
Equations which show how chlorine atoms catalyse decomposition of ozone
Cl* + O3 → ClO* + O2
ClO* + O3 → 2O2+ Cl*
What is the overall reaction for the depletion of ozone
2O3 —–> 3O2 and chlorine free radicals are the catalyst
Why might excess ammonia be used
To prevent further substitution
