13 - Alkenes

Question 1

Define alkene

Answer 1

Unsaturated hydrocarbon containing a C=C bond comprising of a TT bond (sideways overlap of adjacent p-orbitals above & below bonding C atoms) and a sigma bond (overlap of orbitals directly between bonding atoms).

Restricted rotation of TT bond

Question 2

General formula of alkene

Answer 2

C n H 2n

Question 3

Define sigma bond

Answer 3

When 2 s orbitals overlap in a straight line – gives highest possible electron density between 2 nuclei. Single covalent bond.

High electron density means there is a strong electrostatic attraction between nuclei and shared pair of electrons. High bond enthalpy, strongest type of covalent bond

Question 4

What is a pi bond

Answer 4

Sideways overlap of two adjacent p orbitals. 2 parts – 1 above and 1 below the molecular axis, bc p orbitals which overlap are dumbbell shaped.

Much weaker than sigma bond bc electron density spread out above and below nuclei. Electrostatic attraction between nuclei and shared pair of electrons is weaker, so TT bonds have a relatively low bond enthalpy

Question 5

Structure & bonding in alkene

Answer 5

Trigonal planar
120 degrees
C=C unit is planar

• 3 regions of electron density around each C atom.
• 3 regions repel as far apart as possible, so 120 degree bond angle
• all atoms in same plane

Question 6

Define stereoisomer

Answer 6

Compounds with the same structural formula but a different arrangement in space

Question 7

E/Z isomerism

Answer 7

An example of stereoisomerism, in terms of restricted rotation about a double bond, and requirement for 2 different groups to be attached to each carbon atom of the C=C group

Question 8

Cis-trans isomerism

Answer 8

A special case of E/Z isomerism in which 2 of the substituent groups attached to each carbon atom of the C=C group are the same

Question 9

3 isomer vs Z isomerism

Answer 9

higher priority group on …

E = either side (from equator lol), entgegen
Z = together, zusammen

Question 10

Cis vs trans

Answer 10

Cis = same group on either side, E
Trans = same group on same side, Z

Question 11

Reactivity of Alkenes

Answer 11

• much more reactive than alkanes
• relatively low bond enthalpy
• due to presence of TT bond
• TT electrons more exposed than sigma electrons -> TT bond breaks easily -> Alkenes undergo reactions easily

Question 12

Addition reaction of Alkenes with hydrogen

Answer 12

• “hydrogenation of alkenes”
• nickel catalyst
• 423K
• alkene + hydrogen -> alkane

-one H2 molecule per double bond in alkene

Question 13

Addition reaction of Alkenes with halogens

Answer 13

• “halogenation of alkenes”
• rapid addition reactions with halogens chlorine / bromine @ RT
• alkene + halogen molecule -> dihaloalkane

Question 14

Addition reaction of alkenes with hydrogen halides

Answer 14

• alkenes react with GASEOUS hydrogen halides @ RT to form haloalkanes
• if alkene = liquid, bubble hydrogen halide through it.
• alkenes can react with concentrated HCl
• can form diff products
  (explored further in electrophilic addition of alkenes)

Question 15

Addition reaction of alkenes with steam

Answer 15

• “hydration reactions of alkenes”
• alkene + steam -> alcohol
• phosphoric acid catalyst, H3 PO4
• steam ADDS ACROSS double bond
• widely used to produce ethanol from ethene
• 2 possible products

Question 16

How to test for unsaturation?

Answer 16

• reaction of alkenes with bromine
• when bromine water (orange coloured) added dropwise to alkene sample, bromine ADDS ACROSS double bond.
  decolourises -> indicate presence of double bond

Question 17

Define electrophile

Answer 17

An electron pair acceptor

Question 18

Electrophilic addition

Answer 18

It is the mechanism for the addition reaction that alkenes usually take part in to form saturated compounds.

• double bond in alkene represents high electron density (bc of presence of TT electrons)
• high electron density of TT electrons attracts electrophiles
• electrophile: (group of) atoms attracted to electron-rich centre and accents electron pair. Usually positive ion / partial positive charge atom in molecule

Question 19

Define carbocation

Answer 19

Contains a positively charged carbon atom

Question 20

Propene and bromine reaction mechanism

Answer 20

Bromine is nonpolar. When bromine approaches propene, induced dipole is formed.

The bromine atom closer to the C=C is positive, other bromine atom is negative

Question 21

Markownikoff’s rule

Answer 21

• predict formation of a major organic product in addition reactions of H-X to unsymmetrical alkenes
• major product: product with highest carbocation stability.
  Tertiary carbocation = most stable

Question 22

Explain carbocation stability

Answer 22

• Linked to electron donating ability of alkyl groups.
• Each alkyl group donates + pushes electrons towards positive charge of carbocation
• positive charge spread over alkyl groups
• more alkyl groups = more positive charge spread out = more stable ion
• so in inc order of stability: primary (2 H, 1 R), secondary (1 H, 2 R), tertiary (0 H, 3 R)