HALOGENOALKANES Flashcards
Define primary halogenoalkane
One carbon attached to the at in atom adjoining the halogen
Define secondary halogenoalkane
Two carbons attached to the carbon atom anointing the halogen
Define tertiary halogenoalkane
Three carbons attached tot the carbon stoma joining the halogen
What are the two reactions halogenoalkanes can undergo
Substitution
Elimination
Define substitution
Swapping a halogen atom for another atom or groups of atoms
Define nucleophile
Electron pair donor
Give an example of a nucleophile
:OH-
:NH3
CN-
Outline nucleophilic substitution
The nucleophile attack the (delta) positive carbon atom
The carbon has a small positive charge because of the electronegativity difference between carbon and the halogen
The nucleophile replaces the halogen on the alkane
How is the rate of a substitution reaction affected by the strength of the carbon-halogen bond?
The weaker the bond
The easier it is to break
And the faster the reaction
What is the trend in rate of a substitution reaction of halogenoalkanes?
Iodo alkanes substitute the fastest
Fluoroalkanes substitute the slowest
Because C-I bonds have the weakest bond enthalpy
And C-F bonds have the strongest bond enthalpy
Define hydrolysis
Splitting a molecule by reaction with water
Describe water as a nucleophile
Water is a POOR nucleophile
But reacts slowly with haloalkanes in a substitution reaction
Describe the reaction between aqueous silver nitrate and haloalkanes
Halide group combined with silver ion to form a silver halide precipitate
I- + Ag+ -> AgI
Describe the colour of silver ha life precipitates
Which form fastest?
AgI (yellow) forms fastest
AgBr (cream)
AgCl (white) forms slowest
Describe the conditions of nucleophilic substitution with aqueous hydroxide ions
Potassium (or sodium) hydroxide reagent
Aqueous solution
Heat under reflux
Describe the change in halogenoalkane when substituted in aqueous hydroxide ions
Forms an alcohol
Why is the aqueous condition important in nucleophilic substitution with hydroxide ions
Ethanol would trigger an elimination reaction
Outline tertiary halogenoalkane nucleophilic substitution
The halogen breaks away from haloalkane
To form a carbocations intermediate
The hydroxide nucleophile then attacks the positive carbon
Describe the conditions of nucleophilic substitution with cyanide ions
KCN dissolved in ethanol/water mixture reagent
Heating under reflux
Outline the change in rules for naming nitriles
Nitriles have to be at the end of a chain
Start numbering the chain from the c in CN
Include the e (butanenitrile not butannitrile)
Describe the conditions of nucleophilic substitution with ammonia
NH3 dissolved in ethanol reagent
Heating under pressure
Outline nucleophilic substitution with ammonia
The ammonia attack the (delta) positive carbon atom
The ammonia replaces the halogen on the alkane producing a halogen radical
Excess ammonia then reacts with the hydrogen on the positive charged nitrogen
Why must excess ammonia be used in nucleophilic substitution
To minimise a lower yield due to further substitution reactions occurring
Define elimination
Removal of small molecule (often water) from the organic molecule
Outline the conditions for elimination with alcoholic hydroxide ions
Potassium (or sodium) hydroxide reagent
In ethanol
With heat
Outline elimination with alcoholic hydroxide ions
The radical reacts with a hydrogen bonded to a neighbouring carbon to the halogen
The bond between mentioned hydrogen and carbon breaks to form a double bond between carbon carbon atoms
The halogen group forms a negative ion
And H2O
What types of reaction are likely to occur between primary and tertiary halogenoalkanes
Primary tends toward substitution
Tertiary tends toward elimination
What are some of the uses of halogenoalkanes
Chloroalkane a and chlorofluoroalkanes can be used as solvents in dry cleaning
Refrigerants
Pesticides
Aerosol propellants
Why have many uses of halogenoalkanes been stopped?
Toxic
Detrimental effects on the atmosphere
Why is the ozone layer benefitial
Filters out harmful UV radiation
What does ozone in the lower atmosphere do
Pollutes
Contributes toward smog formation
Outline the conditions for nucleophilic substitution of halogenoalkanes with aqueous hydroxide ions
Potassium/Sodium hydroxide
Aqueous solution heat under reflux
Neucleophile OH-
