Alkenes

General formula

CnH2n

What bonds do alkenes contain

Double bonds

Are alkenes saturated or unsaturated

Unsaturated

First member of alkenes

Ethane C2H4

Structural formula of Ethel

H2C=CH2

Structural formular of propone

H2C=CH2CH3

Structural formula of butane

H2C=CHCH2CH2CH3

Why does E/Z isomerism occur

Due to the restricted rotation around the double bond

What happens when orbitals directly overlap

Sigma bonds are formed

When else are sigma bonds formed

When covalent bonds are formed

How do p orbitals overlap when a double bond forms

Sideways

What does the sideways overlap of p orbitals form

Helps form pi bonds

Pi bonds

Second bond of the double bond that locks the structure and prevents further rotation of the molecule

Single covalent bond=

Sigma bond, direct overlap

Double covalent bond=

Pi bond, sideways overlap

What kind of orbitals can overlap

P orbitals only

Shape and bond angles of alkenes

Trigonal planar- 3 areas of electron bonding, 120deg. BAs

BP/MP of alkenes

Similar trends to alkanes, increases as more carbons in formula- more atoms=more LDFs, great IM force= more energy needed to separate molecules, more energy required= higher BP

What is the state of lower member s at room temp

Gases at room temp and pressure

State of cyclohexane for isomers

Liquid- greater branching= lower bp

Solubility of alkenes

Non polar so are immiscible in water( don’t mix), miscible with most organic solvents

Where is electron density found

C=C bond

What does the electron density around the C=C bond mean

Alkenes are more reactive than alkanes

Are pi or sigma bonds more reactive

Pi bonds re more reactive as they are weaker than sigma bonds

What does an increased concentration of e- in pi bonds mean

Likely to attract ny species with a plus or delta + charge

Electrophiles

Species with a + or a delta+ charge

Which molecules have potential to react with double bond

Any polar molecules

What can the concentration of negative charge in the pi bond do

Induce dipoles in no polar molecules and turn them into electrophiles

Bromine water test

Colourless= alkene, no change= alkane

Electrophilic addition mechanism

A pair of electrons from a double bond forms a coordinate covalent bond with A of C2H4+ AB, the AB bond breaks to release anion B, a positively charged intermediate carbocation is formed, a lone pair of electrons in B ion forms a coordinate covalent bond with the positively charged intermediate

What happens wen the alkenes are unsymmetrical

Unequal amounts of each product are formed due to relative stabilises of carbocation intermediates

Primary carbocation

C+ bonded to one other C

Secondary carbocation

C+ bonded to 2 other Cs

Tertiary carbocation

C+ bonded to three other Cs

What are reactions most likely to happen with

Most stable carbocation

Stability’s of carbocation

Primary= least, tertiary- most

Conditions for alkenes reacting with hydrogen

Nickel catalyst, 300-400 deg c

Conditions needed for alkenes to react with water

Steam hydrogenation

Conditions needed for alkenes reacting with chlorine or another halogen

Room temperature, halogen

Conditions for steam hydrogenation of ethane to make ethanol

React with steam at 320 deg c, phosphoric acid (H3PO4) catalyst