Topic 9 Alkenes and Alkynes: further reactions Flashcards
The bromonium ion in the dicolourisation of bromine water when testing for alkenes is …
Br adds to the… while OH added to the …
Attack on the delta positive end of the bromonium ion accounts for both the … and the … addition
The addition is …
unsymmetrical
less substituted carbon
more substituted carbon
regioselectivity
stereoselectivity
SN2
H2O isn’t … to react directly with an alkene. So, the reaction must be catalysed by the addition of an acid by the … of the alkene to form a …
a strong enough acid
protonation
tertiary carbocation
Oxymercuration process:
Alkene reacts with … and … then … in a … step process. … is electrophilic so is attacked by the … .The product of the oxymercuration process contain a … bond rather than a … bond so we must … the bond.
Hg(OAc)2
H2O
NaBH4 (reducing agent)
2
Hg
Nucleophilic alkene
C-Hg
C-H
reduce
Hydroboration is …
Boron is slightly less … than hydrogen, giving the B–H bond a slight … on boron. Boron is very … having only … valence electrons (the B is Lewis acidic). The
nucleophilic alkene can therefore attack the boron
Both bonds are formed … .Boron attached to the … carbon, to give a … carbocation. More bulky boranes show better… in their addition reactions with alkenes indicating … play a role
The addition of borane (BH3) or an organoborane (RBH2 or R2BH) to an alkene.
electronegative
delta positive
electrophilic
six
on the same face
less substituted
more stable
regioselectivity
sterics
An sp hybridised carbon is more … than an sp2 or sp3 hybridised carbon so is much less tolerant of positive charge
electronegative
Instability of vinyl carbocations means
alkynes are reluctant to react with electrophiles and if they do the alkene is often more reactive than alkyne so the reaction tends not to stop at alkene
Chemoselectivity
when a molecule contains more than one functional group, chemoselectivity refers to the preference
displayed by a reagent to react with one of the functional groups over the other(s). For example, in a molecule that contains both an alkene and an alkyne, H2 and Lindlar’s catalyst will chemoselectively reduce the alkyne to an alkene
Regioselectivity
This occurs when a functional group can react in two difference places and refers to the preference of the reagent to react in a particular place. For example, HBr will add regioselectively to an unsymmetrical alkene: the H will end up on the less substituted carbon and the Br will end up on the more substituted carbon, because the reaction proceeds via formation of the more substituted carbocation. The reaction is said to proceed with regiocontrol and the two possible products are known as regioisomers
Stereoselectivity
This is the preferential formation of one stereoisomer over another in a reaction. An example is the preferential formation of the Z-alkene in the reduction of an alkyne by H2 and Lindlar’s catalyst, or the trans addition of bromine across a double bond. The reaction is said to proceed with stereocontrol.
Hydroboration is important as
organoboranes can be oxidised to alcohols by treatment with hydrogen peroxide and sodium hydroxide
Dihydroxylation is the addition of … across a … to make a … . The reaction uses … as the oxidant, forming initially a …; water hydrolyses the to the …, the reaction is performed in a water-miscible solvent such as …
two hydroxyl groups (OH)
double bond
1,2-diol
osmium tetroxide
cyclic osmate ester
osmate ester
diol
acetone or tert-butanol
Ozonolysis is …
Ozonolysis is a … process: reaction with O3 , typically in an inert solvent such as CH2Cl2 at –78 ̊C, gives an ozonide; a second … step is required to yield the carbonyl compounds – normally Zn and acid, or triphenylphosphine (Ph3P) or dimethyl sulfide (Me2S) is added to reduce the ozonide. The ozonide can also by broken down under… conditions with … to produce … rather than …
Reaction with ozone is a very useful way to convert alkenes into aldehydes or ketones. Ozone is made by passing an electrical discharge through oxygen
two-step
reductive
oxidising
H2O2
carboxylic acids
aldehyde products
Epoxidation is …
The most commonly used peracid is …
Reaction of alkenes with peracids converts them into epoxides
mCPBA
Alkenes are readily … to alkanes by … .It is supported on … to give it an …, since this is a … process. If we use an alkene that allows us to see the stereochemical outcome of the reaction, we see that we get cis addition of the two hydrogen atoms. The reaction occurs on the metal surface, and both hydrogen atoms are added to … of the alkene
reduced
hydrogen gas and a transition metal catalyst. Often palladium is used for this
charcoal (denoted Pd-C)
enormous surface area
heterogeneous
the same face
Diels Alder reaction is …
The reaction works best with …
An alkene and a diene react in a so-called cycloaddition process to form a six-membered ring
Electron rich dienes and electron deficient dienophiles
