Halogenoalkanes Flashcards
Name the halogens
Fluorine, chlorine, bromine and iodine
What makes up an halogenoalkane
Alkane skeleton with one or more halogen
How to name halogenoalkanes
Prefixes: Fluoro, chloro, bromo, iodo = which halogen present
Numbers used to show which carbon the halogen bonded to
Di, tri, tetra … used to show how many atoms of each halogen present
Halogens listed in alphabetical order
Polarity of halogenoalkanes
- Halogens F, Cl, Br and I have higher electronegativities than C
- Covalent bonds between carbon and halogens are polar (electrons not evenly distributed)
- The more polar the greater the dipole (I, Br, Cl, F in order)
Stereoisomers
= isomers whose atoms are connected in the same order but have different spatial arrangements
What is a chiral structure
Compound that has at least 1 carbon bound to at least 4 other atoms/groups
Are mirror objects but cant be superimposed
- chiral carbon atom = asymmetrical carbon/ stereo centre
What is an enantiomer
A pair of chiral molecules
What is an achiral structure
Has a mirror and can be super imposed
What is racemate
- a 1:1 mixture of enantiomers (left handed : right handed)
- many drugs are chiral with 1 enantiomer possessing desired properties
- is starting materials achiral and products chiral it is a racemic mixture
- opposite enantiomer may be biologically inactive, active in a different way or produce undesired effects e.g. thalidomide
What are the different ways to describe enantiomers
- draw it
- use R and S system
- Use +/- system
- use d/l system
Explain how to use the R and S system
- way to describe enantiomers
1. Identify the stereogenic centre
2. Assign priority number to each group - atoms with higher atomic number get higher priority
- if 2 atoms are the same then assess the atoms attached to them
3. Arrange the molecule so that the lowest priority group is pointing away
4. Follow 1,2,3 around the circle - clockwise = R
- anti clockwise wise = S
Explain how to use the d/l and +/- minus systems
Used to describe enantiomers and detect their differences
- polarised light is rotated in different directions by enantiomers
- clockwise = d or +
- anti clockwise l or -
What are diastereoisomers
Stereoisomers that dont have mirror images
What are the 2 different reactions of halogenoalkanes
- nucleophilic substitution
- elimination
What is a nucleophile
Chemical species that donates a pair of electrons
What are the 2 mechanisms within nucleophilic substitution
Sn1 and Sn2
Describe the Sn1 mechanism
- leaving group goes first and then the nucleophile joins later (heterolytic dissociation)
- atom leaves halogenoalkane = unstable
- nucleophile then attacks and binds
2 transition states
Rate of reaction depends on the first slow step = reaction rate is 1st order
Nucleophile can join at the top or bottom and will become a different form of stereoisomer
Describe the Sn2 mechanism
- leaving group goes and nucleophile joins at the same time
- only 1 transition state
- rate of reaction depends on nucleophile and carbon skeleton = rate of reaction 2nd order
- Walden inversion takes place
Describe elimination reaction
- nothing replaces the lost atom
- an alkene is formed
- the nucleophile attacks the H instead of the C
E1 = 2 step reaction E2 = 1 step reaction
When is Sn1 used and when is Sn2 used?
It depends on the hyperconjugation of the carbon skeleton
- tertiary is the most stable = SN1 reaction
- primary = SN2 reaction
When is elimination used and when is substitution used?
- depends on the nucleophile and how strong it is
- strong = elimination
- weak = substitution
