Organic I - electrophilic addition of alkenes & alkynes

What is the standard test for alkenes (C=C)?

Decolourisation of bromine water

How is it proven that electrophilic addition of halogens is not concerted?

• If concerted it would add the bromines to the same side of the molecule

• Bromines add to opposite faces of the molecule

What is the mechanism of electrophilic addition of a halogen to an alkene?

• Occurs with Br2

• Attack from the double bond and bromine molecule form a bromonium ion intermediate

• Bromonium intermediate is broken by attack from the other bromide ion (Br-)

• Causes addition to opposite faces of the molecule

How do different alkene stereoisomers give different products upon addition with halogens?

• E-alkenes may give meso products (opposite planes of the molecule) - different chiral orientations

• Z-alkenes may give enantiomers (same plane of the molecule) - same chiral orientations

How is bromination of alkenes stereoselective?

Geometry of the starting product determines which isomer is obtained as the product

How could syn-addition occur during bromination?

• In a species that can stabilise a carbocation as an intermediate species (eg phenyl)

• Majority will still follow anti-addition

How does the secondary attack location change the final product?

• Attack on the top face of a carbocation leads to syn-addition

• Attack on the lower face of a carbocation leads to anti-addition

• Attack directly on the bromonium intermediate leads to anti-addition

How can alkene substituents affect the rate of reaction?

• Formation of the bromonium ion is the rate determining step

• Electron donating groups (+I) aid bromonium ion formation - increases rate

• Electron withdrawing groups (-I) disfavour bromonium ion formation - decreases rate

How does reaction with other halogens change the reaction?

• Cl2: less stereoselective - unlikely to form a cyclic intermediate - carbocation intermediate is more stable

• F2: highly exothermic and leads to bond cleavage

• I2: does not directly occur and is reversible

How can alkynes be brominated?

• React with Br2 in a non-polar solvent (eg CCl4)

• Gives mostly anti-addition (some syn)

• React in a similar way to alkenes - form an alkene product

How is interhalogen addition with alkenes regioselective?

• More substituted end of the double bond is more able to stabilise a delta+ charge

• Nucleophiles generally attack at the more hindered side of the bromonium ion

How do dienes react with bromine?

• Two possible products (more substituted and less substituted)

• More substituted is reacted with 1 equivalent of Br2 and heat - thermodynamic product

• Less substituted is reacted with an excess of Br2 under low temperatures - kinetic product

How do hypohalous acids react with alkenes?

• Forms a bromonium intermediate - -OH acts as the leaving group

• H2O acts as the nucleophile

• Forms a bromohydrin

Why are the more substituted OH locations on bromohydrins favoured?

• Asymmetrical bromonium ions open regioselectively

• There is a more electrophilic site at the substituted end due to +I

• Bond from the more electrophilic site to the Br is longer and weaker

How can bromohydrins be used synthetically?

• Can be used to make epoxides

• Add base (eg NaOH) which allows the reaction to begin

How do hydrogen halides add to alkenes?

• Double bond attacks the electrophilic hydrogen, bromine leaves

• Carbocation intermediate is formed

• Bromide ion attacks the positively charged carbon to form the haloalkane

How does bromine react with asymmetrical alkenes?

• Will favour production of the more stable carbocation intermediate (3 > 2 > 1)

• More likely to form tertiary or secondary bromide

What is usual addition of hydrogen halides to alkenes called?

Markovnikov addition (goes via most stable carbocation intermediate)

What are the conditions for addition of a hydrogen halide to an alkene?

• Excess of HX

• Polar solvent (eg ethanoic acid) - helps with HBr dissociation and stabilisation of the carbocation

What is the conditions for reacting a hydrogen halide with an alkene using free-radical chemistry?

• Use a radical initiator (tBuO-OBut) and heat

• Radical propagation will form a Br radical

• Br radical will add to the alkene to form the most stable radical intermediate

• Radical will then attack an H-Br to propagate the reaction

What are the consequences of free-radical addition to an alkene?

• Halogen ends up attached to the least substituted carbon atom

• Hydrogen ends up attached to the most substituted carbon atom

• Orientation of addition is sometimes called anti-Markovnikov addition

How does polar addition of HBr to dienes occur?

• Leads to a rearrangement of the molecule

• Forms the most stable carbocation

• Br attacks at the least hindered position on the carbocation (usually terminal)

• Rearrangement of the double bond will produce the thermodynamic product

How can water be added using electrophiles to an alkene?

• If the substrate can produce a tertiary carbocation an aqueous acid can be used to effect hydration - use a mineral acid with a non-nucleophilic counter ion

• Reliable method utilises Hg2+ complexes (oxymercuration) - forms a mercurinium ion intermediate

• The ion intermediate is reactive towards ring-opening at the most substituted position with a nucleophile - forms a tertiary/secondary alcohol

• Goes via Markovnikov addition

How can oxymercuration be used with an alkyne?

• Forms a ketone instead of an alcohol

• Forms a mercurinium ion and enol intermediates

• Mercury is lost in acidic conditions