U2 KA2- Organic synthesis Flashcards
Double headed curly arrows
Used to indicate the movement of electrom pairs in a reaction
* tail is where the electrons orginate from
Single headed curly arrows
Indicates the movement of a single electron
Heterolytic bond fission
- occurs when a covaent bonod between 2 chemical species is broken in an unequal manner
- bond pair of electrons being retained by one of the chemical species
Homolytic bond fission
- occurs when the bond is non polar or very slightly polar
- one electrom from bond goes to one atom while the other electron goes to the other atoms
Production of free radicals
- chain initiation- UV light makes Br into free radicals
- chain propogation- one radical enters reaction and one is formed
- chain termination- remove free radicals from reaction ithout replacing new ones
How are ions formed?
by heterolytic fission
(favoured when the bond is polar)
Electrophile
positively charged ions
can accept electron pair
Nucleophiles
Negatively charged ions
at least 1 lone pair of electrons
electron dontating
Haloalkanes
Also known as halogenoalkanes
one or more H replaced with a halogen atom
What is the difference between primary, secondary and tirtiary monohaloalkane?
Primary- one alkyl group attached to the carbon atom directly atached to the halogen atom
Secondary- 2
Tirtiary- 3
Why are haloalkanes susceptible to nucleophilic attack?
the presence of the slight positive charge on the carbon atom (polar nature)
Nucleophilic substitutions
Reactions of monoalkanes with alkalis to produce alcohol
* Potassium hydroxide (KOH) or Sodium hydroxide (NaOH) is used
* reaction with alcoholic potassium alkoxide produces ethers
* reaction with ethanolic potassiym cyanide or sodium cyanide (KCN/NaCN in ethanol) produces nitriles increases chain length and nitrile can be converted into carboxylic acid through acid hydrolysis
Sn1
first order nucleophilic substitution
Sn2
Second order nucleophilic substitution
What is formed in the Sn1 reaction?
A carbocation intermediate (tirtiary is most stable)
positively charged intermediate
What is formed in a Sn2 reaction?
negatively charged intermediate is formed
How are elimination reactions achieved?
When monohaloalkanes undergo elimination reactions to form alkenes
* heating monohaloalkane under reflux with ethanolic potassium hydroxide or sodium hydroxide
What decides if you get sub or elim reaction
sub- if the nucleophile is in aqueous solution (in water)
elim- if the nucleophile is in ethanol
How are ethers synthesised?
- from haloalkanes and alkoxides
How are ethers named?
- by assigning the longest carbon chain as the parent name
- prefixed by rhe alkocy sub (tke away -yl and add -oxy)
such as ethoxypropane, methoxypentane etc
Properties of ethers
- bp lower than alcohols (no hydrogen bonding)
- low relative formula mass
- soluble in water (smaller ones only)
- highly flammable
- volatile
Electrophilic addition
- alkenes undergo these with a variety of substances
Hydrogenation
- catalytic addition of hydrogen
- cataclysed by nickel or palladium
Halogenation of alkenes
- Br is the electrophile and undergoes heterolytic fission
- polarises at the double bond
- forms a intermediate bromonium ion
- Br ion attacjs intermediate and is acting as the nucleophile
- breaks delocalised ring to form haloalkane
Hydrohalogenation
- addition of hydrogen halides to alkene
- draw and check on slide 81 of U2 powerpoint
Markovniov’s rule
its not very catchy !!
When H-X is added onto an unsymetrical alkene the major product is the one where the hydrogen bonds to the carbon atom of the double bond that already has the greatest numberr of hydrogen atoms attached to it
Hydration
- addition of water to alkene
- step one produces a carbocation
- step 2 the carbocation undergoes nucleophilic attack by a water molecule to give protonated alcohol
- step 3 the protonated alcohol loses a proton to give final alcohol
