Alkenes

What bond defines and alkene?

C=C bond

What is the shape and boding angle around a C=C bond?

planar 120 degrees

which bond is stronger sigma or pi bond?

sigma bond

What bond(s) does C-C have?

sigma

What bond(s) does a C=C bond have?

sigma and pi

Why is there no rotation around a C=C bond?

• The π(pi) bond is formed by overlap of p orbitals

• If there was any rotation, the p orbitals would no longer overlap and the π bond would break

How is a C=C bond made?

made of two shared pairs of electrons (double covenant bond)

Which is E and which is Z isomerism ?

Left is Z and right is E

Electrophile definition?

electron pair acceptor

Why will C=C bonds break fairly easily?

C=C bonds are areas of high electron density and the π bond is weak so it will break easily

What are electrophiles attracted to?

the C=C double bond and will react with alkenes

What are the three electrophiles that react with alkenes?

• Hydrogen halides (e.g. HCl, HBr)

• Halogens (e.g. Cl2, Br2)

• Sulphuric acid (H2SO4)

mechanism name to form a haloalkane from an alkene?

Electrophilic addition

Reagents to form a haloalkane from an alkene with hydrogen halides?

HCl or HBr

Reagents to form a haloalkane from an alkene with halogens?

Cl2 or Br2

Conditions to form a haloalkane from an alkene?

None

draw and name the mechanism for the reaction of C2H4 to form C2H5Br

electrophilic addition

draw and name the mechanism for the reaction of C2H4 with Br2

electrophilic addition

What is the reactive intermediate in electrophilic substitution called and why?

a carbocation, because it is a positive molecular ion (cation) and the charged atom is carbon

What does the stability of a carbocation depend on?

The number of alkyl groups bonded to the positive C atom

What is the positive inductive effect?

The ability to release electron density through a covalent bond

(When alky groups push electron density onto whatever they are bonded to)

Why are tertiary carbocations more stable than primary carbocations?

There is a greater positive inductive effect from three alkyl groups in 3° carbocations compared to two alkyl groups in 2° carbocations

Between 2-bromo propane and 1-bromo propane, why is 2-bromo propane the major product?

• 2-bromopropane is made from a secondary carbocation

• 1-bromopropane is made from a primary carbocation

• Secondary carbocations are more stable than primary carbocations due to the positive inductive effect of the two alkyl groups (compared to one)

Why does bromine react with the double bonds in alkenes even though it is a non-polar molecule?

• The C=C bond is electron rich

• and induces a dipole in the Br2

• δ+ Br attracted to C=C double bond

Chemical test for alkenes

Bromine water turns from orange to colourless if alkene is present

mechanism name to form an alkyl hydrogen sulphate from an alkene?

electrophilic addiction

reagents to form an alkyl hydrogen sulphate from an alkene?

H2SO4

Conditions to form an alkyl hydrogen sulphate from an alkene?

None

Draw and name the mechanism for the reaction of H2SO4 with C2H4

electrophilic addition

Draw displayed and structural formula for H2SO4

How do you name a Hydrogen sulphate?

Name the carbon chain as an alkyl chain (methyl, ethyl etc.) and end the name in ‘hydrogensulphate’.

What is this molecule called?

Ethyl hydrogensulphate

What is made if a hydrogen sulphate is reacted with water?

an alcohol

What are the general equations for making an alcohol from an alkene in a 2 step process?

Step 1

• alkene + H2SO4 à alkyl hydrogen sulphate

Step 2

• alkyl hydrogen sulphate + H2O à alcohol + H2SO4

What is the role of the H2SO4 in the two-step transition from alkene to alcohol?

catalyst

Draw and name mechanism for alkene to alcohol in one step?

Direct hydration

Reagents for alkene to alcohol in one step?

H2O

Conditions for alkene to alcohol in one step?

Conc. H3PO4 catalyst

What is the product of direct hydration?

an alcohol

What is an addition polymer?

a long chain made from lots of monomers (alkenes joining together)

How can alkene monomers join together?

by breaking the π bond in their C=C bond and using the pair of electrons from this bond to form a bond to another alkene

Write an equation for the formation of an addition polymer from ethene.

how to draw a repeating unit?

repeating unit must always be shown with bonds extending through the brackets

How can the properties of the plastics produced be altered?

through the alteration of the reaction conditions used in the production of the polymer chains

How are polymers with weak intermolecular forces produced?

High pressure and temperature produces branched chain polymers

What bonding do branched chain polymers have?

weak intermolecular forces

How are polymers with strong intermolecular forces produced?

lower pressure and temperature produces straight chain polymers

What bonding do straight chain polymers have?

strong intermolecular forces

Which have stronger bonds? branched or straight chain polymers

straight chain polymers have strong intermolecular forces

Are polymers reactive?

NO they are unreactive

Why are polymers good for manufacturing everyday plastic products?

They are unreactive hydrocarbon chains with strong non polar covenant bonds

What is the con of polymers being unreactive?

they are not biodegradable

What does PVC stand for

poly(chloroethene)

What properties does PVC gain at the addition of plasticiser and why?

is more flexible as they slide between the polymer chains pushing them apart?

Name this polymer

poly(ethene)

Why are polymers not reactive?

the carbon chain is saturated and the side chains are usually non-polar

Give an equation, showing structural formulas, for the conversion of chloroethene into poly(chloroethene).

State what you would observe if bromine water was added to a polymer?

orange solution as the polymer is unsaturated

what can plasticiser with PVC used for?

for electrical cables

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