2.4 - Alkenes

Define sigma bond

covalent bond formed by linear overlap of atomic orbitals

Define Pi bond

covalent bond formed by sideways overlap of p orbitals

Define bond length

distance between nuclei of two covalently bonded atoms

Define hydrogenation

addition of hydrogen molecule across C=C

Define electrophile

ion/molecule attacking regions of high e- density

Define heterolytic fission

bond breaking where both e- in shared pair go to single atom

Define primary/secondary/tertiary carbocation

carbocation with 1/2/3 carbon atoms directly bonded to +ve charged carbon

Define polymerisation

joining small molecules forming large molecule

Define monomers

small molecules joining forming polymer

Define polymer

large molecule formed when monomers join

Which alkenes have structural isomers and why?

alkenes with 4+ carbons as different positions of C=C and branching

Which alkenes are gases at room temp?

C2 to C4

Which alkenes are liquids at room temp?

C5 to C16

Which alkenes are at solids at room temp?

C17 and above

Why does length of carbon chain influence alkenes state at room temp?

bpt increase with RMM/VDW strength

Why are branched isomers more volatile(lower bpt)?

less contact between molecules so lower attraction

Are alkenes polar or non-polar?

non-polar

Shape of an alkene molecule

trigonal planar shape

What prevents free rotation in alkene molecules?

C=C

Key features of E-Z isomer

C=C, each C in C=C bonded to two different atoms/groups

Bonds in a carbon-carbon double bond

sigma bond by linear overlap of atomic orbitals and pi bond by sideways overlap of adjacent p orbitals above/below sigma bond

Why are alkenes more reactive that alkanes?

C=C have higher bond energy/shorter than C-C.

Why are electrophiles attracted to C double bond?

C=C has region of high e- density attracting electrophiles

What reactions do alkenes undergo to lose double bond?

alkenes unsaturated and undergo electrophilic addition reactions

Reagent for hydrogenation

hydrogen gas

Conditions for hydrogenation

finely divided nickel catalyst, 180*, 4atm

Equation for hydrogenation

C2H4 + H2 = C2H6

What is manufactured via hydrogenation?

margarine

What test is bromination?

test for unsaturation

Reagent of bromination

bromine water

Conditions for bromination

room temp

Observations for bromination

decolorisation of bromine water from orange to colourless

Equation of bromination

C2H4 + Br2 = C2H4Br2

Reagent for halogenation by hydrogen halide

hydrogen halide

Conditions for halogenation by hydrogen halide

room temp and if HCl catalyst required

Equation for halogenation by hydrogen halide

C2H4 + HBr = C2H5Br / C2H4 + HCl = C2H5Cl

What do unsymmetrical alkenes(propane onwards) produce when reacted with hydrogen halide?

two isomeric products so mixture of isomers produced

How do you explain the formation of the major product in asymmetric addition?

reaction occurs with most stable carbocation intermediate

What is the mechanism for halogenation?

electrophilic addition mechanism as C=C is e- rich/attracts electrophiles

What do alkyl groups do to carbocations?

stabilise so tertiary more stable than secondary and primary so halogen atom attacks most readily the tertiary carbocation

What conditions addition polymerisation carried out under?

heat/high pressure, catalyst