3.3.3 - Haloalkenes Flashcards
What are three types of attackers
Nucleophile
Electrophile
Free radicals
Explain what a nucleophile is, what it attacks and examples
Posses a lone pair of electrons (with which to form a bond )
Attack positive and delta positive
Examples :OH- , :CN- , :NH3
Explain what a electrophile is, what it attacks and example
Electron deficient short of electrons therefore has a positive charge or delta positive
Attacks electron rich areas (e.g. C=C)
Examples H+ , H delta +
Explain what free radicals are, what they attack and examples
Has an unpaired electron (very reactive )
Attacks everything e.g. C-C
Examples Cl• , Br•, •CH3
Describe single , non-polar bonds
No delta + or delta -
Therefore only attacked by free radicals
Eg alkanes
Describe single , polar bonds
An electronegative element causes C delta + therefore attacked by nucleophiles
Eg haloalkanes
Describe double bonds
C=C contains electron rich area therefore attacked by electrophile
Eg alkenes
Three types of reactions
Substitution
Elimination
Addition
Explain substitution
Single bonds one atom or a number of atoms is replaced by another
C-X +Y – C-Y + X
Explain elimination
Single bonds small molecules is removed from a larger molecule a single bond becomes a double bond.
XC-CY – C=C + XY
Explain addition
Double bonds a molecule is added to a molecule with double bonds a double bind becomes a single bond
C=C + XY – XC=CY
What do curly arrows represent
Movement of electron pairs
Why are haloalkanes attacked by nucleophiles
Halogen atoms are more electronegative than C so C become delta positive
What conditions are needed for NaOH to nucleophiliclly substitute a haloalkanes
Warm , aqueous
Nucleophilic substitution What happens when NaOH reacts
Replaced the halogen with OH group and produces a NaX compound
Nucleophilic substitution what condition are needed for KCN
Ethanolic, warm
Nucleophilic substitution what happens when KCN reacts
Replaces halogen with CN group makes a nitrile and KX
Nucleophilic substitution what conditions are needed for NH3 to react with haloalkane
Excess in ethanol and high pressure
Nucleophilic substitution what happens when NH3 reacts with haloalkane
NH3 replaces halogen then further on it becomes NH2 which makes an amine
How do you stop further substitution occurring in nucleophilic substitution with NH3
Use excess NH3
What conditions are needed for a elimination reaction with NaOH
Alcoholic and reflux in alcoholic solution
What happens in elimination when alcoholic NaOH is used
Removes a H atom and a halogen atom makes an alkene
Why does the NaOH have to be alcoholic to make elimination occur
In alcohol the :OH- acts as a base and picks up protons
Show diagram of elimination of haloalkanes
| |
- C - C -
| |
H X
/
/
OHWhat is initiation in radical reaction mechanisms
Molecules are exposed to UV light breaks apart to make radicals
What happens in propagation of radical reaction mechanisms
Step 1
Alkane/Haloalkane + Haloradical makes a carbon radical and a HF (etc)
Step 2
Carbon radical + halomolecule make alkane haloalkane + haloradical
What happens in the termination stage of radical reactions
Radicals react with each other to form stable compounds
What is the ozone layer and what does it do
O3 formed naturally in upper atmosphere
Absorbs harmful UV radiation
What are CFC’s
There decomposing characteristics
Chlorofluorocarbons
Very unreactive
Reach ozone layer before decomposing
What has research showed us about CFCs
As we started using CFCs the ozone has depleted
Alternative to CFC’s
HCFCs
Degrade before atmosphere
HFC
No Cl so no ozone depletion potential
What is the mechanism that causes ozone depletion
Free radical substitution
What is the initiation step for breaking down the ozone
C-Cl bonds are weakest bonds in compound so are broken
UV radiation homolytically produces free radicals
CF3Cl —- CF3 +•Cl
What are the propagation steps of ozone depletion
Chlorine radicals react with O3 to make ClO radicals and oxygen molecules
O3 + •Cl —- ClO• + O2
Chlorine radicals then react with O3 to produce oxygen and chlorine radicals
ClO• + O3 —- 2O2 + Cl•
What is the termination step of ozone depletion
Eventually 2 radicals react with each other which will stop the reaction
Cl•+Cl• —- Cl2
