organic chemistry - halogenoalkanes and alkenes Flashcards
(44 cards)
what do halogenoalkanes contain?
at least one halogen atom in the compound. this could be F, Cl, Br, I
How are halogenoalkanes formed?
from alkanes in free radical substitution
when do the boiling points increase in halogenoalkane?
the significant increase in size of the halogen atoms (increase in Mr) causes greater van der waal forces which increase the boiling point
what bonds do halogen alkanes contain?
polar bonds
are halogens soluble in polar solvents (e.g. water?)
generally not soluble/ miscible in polar solvents
are halogens soluble in non-polar solvents (e.g. hexane or other hydrocarbons?)
they are soluble in non-polar solvents.
this is particularly true for longer chained molecules
what is a nucleophile?
electron pair donor
most common nucleophiles:
- CN:
- OH:
- NH3
what’s nucleophilic substitution
changing functional group
a mechanism that shows how nucleophiles attack halogenoalkanes . produces alcohols or amines
conditions and reagents for nucleophilic substitution with NaOH
- NaOH in aqueous solution
- heat gently and reflux
conditions and reagents for nucleophilic substitution with KCN (potassium cyanide)
- KCN in aqueous solution
- heat gently and reflux
conditions and reagents for nucleophilic substitution with NH3 (ammonia (non-charged nucleophile))
- concentrated ammonia in excess
- heat
- in a sealed container under pressure
how does NH3 act in nucleophilic substitution
NH3 is a base (it accepts H+ ions)
hydrolysis of halogenoalkanes
C-I is the easies to break compared to C-Br and C-Cl . therefore its hydrolyses fastest
what is the role of ethanol with halogenoalkanes
it dissolves halogenoalkanes and mixes with water, used in elimination reactions
What is elimination? And whats its products for halogenoalkanes?
A mechanism that involves the removal of an atom or group of atoms. Halogenoalkanes undergo elimination to form alkenes (C=C)
when does elimination occur?
when a halogenoalkane is heated to high temp under alcoholic conditions
how does the nucleophile act in an elimination reaction?
acts as a base and accepts a proton, removing a hydrogen atom form the molecule. this results in the elimination of the halide too producing a carbon= carbon
The mechanism for elimination
- OH- acts as a base to remove H from an adjacent carbon (next to)
- Electrons in C-H bond, move to C-C bond to form an alkene
- C-Cl bond breaks to give electrons to the Cl
What is the functional group of alkenes and what does this limit?
C=c
This double bond has limited rotation and this gives rise to stereoisomerism
How can you work out if an alkene is Z or E
The “priority group” is chosen based on the proton (atomic) number of the adjacent atom
Structure for Z
Two important parts are together
Structure for E
Two important parts are opposite
Alkenes
General formula
Functional group
Cn H2n
C = C
