organic chemistry - halogenoalkanes and alkenes Flashcards
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
Melting and boiling points of alkenes
They are low due to weak intermolecular forces , van der walls, between molecules
Similar to alkanes but a little higher
Polarity of alkenes
Non-polar molecules as there’s no electronegativity difference between C’s
Polymerisation of alkenes
where many monomers join to form long chained polymers. the double bond is broken to form a repeating unti
E.g. ethene -> poly(ethene)
reaction conditions for the production of polymer chains
- high pressure & temp - produces branched chain polymers with weak IMF
- low pressure & temp - produces straight chain polymers with strong IMF
What are polymers
unreactive hydrocarbon chains with multiple strong, non-polar covalent bonds.
though due to them being unreactive they are not biodegradable
what is PVC
poly(chloroethene) is an addition polymer with waterproof properties because of the addition of plasticisers during the reaction
Whats an electrophile
An electron pair acceptor
what are common electrophiles?
- HBr
- Br2
- H2SO4
electrophilic addition
breaks the double bond and a carbocation forms. this is a carbon atom with 3 bonds so it has a + charge
what can electrophiles be used to form?
- alkyl hydrogensulphates
- halogenoalkanes
Definition of addition
Adding into a compound (loss of a double bond)
Electrophilllic addition
Ethene plus H-Br
1) polar H-Br approaches e- rich C=C
2) electrons from C=C attack H (delta +) to form new C-H bond, breaking H-Br bond
3) this forms a carbocation (positively charged ion) and Br-
4) lone pair form Br- forms new bond to C+
What is addition with asymmetric alkenes
When there are different “options” as to which carbon forms the first bond, and which has the positive charge
How can you figure out what product is major or minor?
They are due to the different carbocations that form and their stability
Primary carbocation (1 carbon is adjacent) - least stable
Secondary carbocation (2 adjacent carbons)
Tertiary carbocation (3 adjacent carbons) - most stable
How can a product be major or minor?
Because of the “inductive effect” of other carbons
Adjacent carbons push electron density towards the positive charge to stabilise it
what does ozone do?
o3, forms a layer in our atmosphere and absorbs uv radiation. its becoming depleted due to CFC
what do CFC’s do?
chloro-fluoro carbons absorb uv radiation, breaking down carbon- halogen bonds to form free radicals that can catalyse ozone depletion into oxygen
what are we doing to prevent ozone depletion and global warming
CFC-free solvents are now being produced to prevent them entering the atmosphere
what alternatives do we use instead of CFC
we use hydrofluorocarbons as they do not generate Cl radicals. they’re used in cool and refrigerants and propellants
what does bromine water do when present with an alkene?
turns the solution from orange to colourless
