halogenoalkanes
No description yet. This curated deck is ready for daily study.
Card preview (24)
why do we aim to get an alkane into a halogenoalkane?
alkanes are unreactive due to; non-polar bonds (very similar electronegativities) so overall non-polar
what is the mechanism for turning an alkane into a halogenoalkane?
free radical substitution
what gets substituted in free radical substitution?
a hydrogen atom for a halogen (Cl/Br)
what are the reagents and conditions for free radical substitution?
reagent; chlorine or bromine mostly- don’t use flourine conditions; UV light (doesn’t possess enough energy to break flourine bond so we don’t use flourine)
what kind of fission occurs in free radical substitution?
homolytic - each atom receives 1 electron each THIS IS THE ONLY HOMOLYTIC MECHANISM IN AS CHEMISTRY
what are the four steps of free radical substitution?
• Initiation • Propagation 1 • Propagation 2 • Termination
outline the aims of each step of free radical substitution?
• Initiation- formation of the radical • Propagation 1 - formation of the HX and radical • Propagation 2- formation of the haloalkane • Termination- removal of radical
what are CFCs and what do they do?
chloroflourocarbons lead to the depletion of the ozone layer
what did scientists discover about waste CFCs?
they didn’t break down easily and would rise up to the upper atmosphere and break down to form chlorine radicals these chlorine radicals catalyse the breakdown of ozone, and have led to a hole in the ozone layer
how does ozone form?
O2 + O* ———> O3 (* represents radical)
why is the ozone layer beneficial?
it absorbs UV radiation that can cause skin cancer
how do chlorine radicals catalyse the breakdown of ozone ?
Cl* + O3 ———> O2 + ClO* ClO* + O3 ———> 2O2 + Cl* Cl* reacts and is regenerated (never used up ) i.e. its a catalyst
what is the overall equation for depletion of the ozone?
2O3 ———–> 3O2
what are alternatives to CFCs?
HFC’s (hydroflourocarbons) that don’t contain chlorine or HCFC’s (hydrochloroflourocarbons)- less stable than CFCs so break down easily (less likely to form radicals) but are banned in USA and UK
what can’t we use in the UK and why?
CFC’s and HCFC’s because they contain C-Cl bonds and a chlorine radical can form
what are nucleophiles?
electron pair donors
what are the three nucleophiles we need to know?
:OH- (forms alcohol) :NH3 (forms amine) :CN- (forms nitrile)
what basically happens in nucleophilic substitution?
replacing halogen for nucleophile
explain why haloalkanes are reactive? what is the exception?
they are polar polarity is due to the difference in electronegativity between carbon and halogen non-polar C-I bond because electronegativities are similar due to increased shielding of iodine
what kind of fission is present in nucleophilic substitution?
heterolytic - electrons distributed unevenly when the covalent bond breaks
what is the process called for nucleophilic substitution for alcohols?
hydrolysis- adding water to break a bond halogenoalkanes react with aqueous hydroxides to form alcohols- known as hydrolysis
what are the reagents and conditions for nucleophilic substitution of alchohols?
reagent; sodium hydroxide solution conditions; warm, aqueous
what are the reagents and conditions for nucleophilic substitution of nitriles?
reagent; KCN (potassium cyanide) conditions; ethanol, warm
what is important to remember about reacting a halogenoalkanes with cyanide?
increases the length of the carbon chain