4.1.3 Alkenes

Are alkanes saturated or unsaturated?

unsaturated

What is the functional group in alkenes?

C=c

What is the general formula of alkenes?

CnH2n

What bonding exists between alkene molecules?

induced dipole-dipole

Where do we source alkenes from?

• crude oil

• cracking

• elimination reaction

What are alkenes used for?

• plastics

• laquers

• detergents

• fuels

in a prop-1-ene what is the bond angle of C1?

120*

in a prop-1-ene what is the bond angle of C3?

109.5*

What is a hydrocarbon?

a compound that only consist of hydrogen and carbon

What are C=C bonds made of?

• one sigma bond

• one pi bond

Describe the rotation of a C=C bond

restricted rotation

In a C=C bond, how is the sigma bond formed?

direct overlap of orbitals directly beyween bonding atoms

How is a pi bond formed?

fouble, sideways overlap of adjacent p orbitals above and below the bonding C atoms

Draw a diagram to show a C=C double bond

Draw a diagram to show the formation of a pi bond

Explain the shape around each C atom in a C=C double bond

• 120* / trigonal planar

• three bonding regions and no lone pairs of electrons

• bonding regions repel each other equally

• all of the atoms are in the same place

Define stereoisomers

compounds with the same structural formula but with a different arrangement of atoms in space

Where does E/Z isomerism occur?

molecules containing a C=C bond

Why does E/Z steroeisomerism require a C=C bond?

it can’t rotate so the atoms attatched to the C=C are fixed in space

What is the criteria for a compound to show E/Z steroisomerism?

• most have a C=C bond

• each C of the C=C bond must have two different groups attached to it

What is the criteria for a compound to be a Z isomer?

the groups with the highest prioority are on the same side of the double bond

What is the criteria for a molecule to be a E steroisomer?

the groups with higher priority are on diagonally opposite sides of the double bond

Who set out the rules to classify stereoisomers?

• Sidney Cahn

• Kelt Ingold

• Vladimir Prelog

When were the rules to classify steroisomers set out?

1951

How are groups on the C=C bond assigned priority?

higher atomic number = higher priority

If the 2 atoms attatched to a carbon atom in a double bond are the aame what must you find?

the first point of difference

For molecules where the atoms attatched to the carbon atom in a double bond is the same how do you assign priority?

the higher atomic number a the first point of difference is given higher priority

Out of CH2CH2OH and CH2CH2Cl which has highest priority?

CH2CH2Cl

What are cis/trans isomers?

special type of E/Z isomerism

What is the criteria for a compound to show cis/trans isomerism?

• must have a C=C bond

• each carbon of the C=C bond must be attached to 2 different groups

• two groupd on the C=C bond must be identical

If there are 2 hydrogen atoms in the steroisomer then will the cis isomer be Z or E?

Z

If there are 2 hydrogen atoms in the steroisomer then will the trans isomer be Z or E?

E

Which nomenlature should be used for steroisomerism by preference?

E/Z

Compare the reactivity of alkanes and alkenes

alkenes are more reactive than alkanes

WHy are alkenes more recative than alkanes?

C=C bond

What types of reactions do alkenes undergo?

addittion

What happens in an addition reaction?

a group is added across the C=C double bond

Which bond breaks in an addition reaction an why?

pi bond

How many products are formed in addition reaction?

1

What is the atom economy of an addition reaction and why?

100% - there are no waste products

What is the chemical test for a C=C group?

add bromine (or bromine water)

What is a positive result for C=C group?

bromine is decolourised

Write an equation for the testing/ positive result of C=C test

CH2=CH2 + Br2 → CH2BrCH2Br

What is the reagents needed to form alkane from alkene?

Hydrogen gas (H2)

What are the conditions needed to react hydrogen and alkene?

• Nickel catalyst (Ni(s))

• 150*C

What reagents are needed to form hydrogen halide (and BR2) from alkane?

HX or X2

What conditions are required to react hydrogenhalide (and BR2) and alkane?

• no catalyst

• room temperature

What are the reagents to form an alcohol from an alkene?

steam

What are the conditions to react steam and alkene?

• high temp (>100*)

• high pressure

• concentrated phosphoric acid

What mechanism d alkenes undertake when reacting?

electrophilic addition

What is an electrophile?

an electron pair acceptor

Why do alkenes attract electrophiles?

the C=C bond is a region of high electron density

How is the electrophilic addition mechanism shown?

curly arrow model

What does a curly arrow show in a model?

the movement of an electron pair to either break or make an electron pair

Draw the electrophilic addition mechanism for ethene an hydrogen bromide

Explain/ describe the electrophilic addition of ethene and hydrogen bromide?

• HBr is a polar molecule bcause Br is more electronegative than H and there is a permanent dipole

• H acts as an electrophile (of HBr)

• H of HBr is attracted to electron dense pi bond

• H accepts a pair of electrons from pi bond

• pi bond breaks by heteolytic fission

• CH bond forms, HBR bond breaks

• carbocation and Br-

• combine to form the organic product

Why is HBr a polar molecule?

Br is more electronegative than H

Which bit of the HBr acts as an electrophile?

H

Where does H of HBR accept an electron from?

pi bond

When H accepts electron pair from pi bond what bonds are formed and broken

CH bond formed

HBr bond broken

What are the intermediate products of electrophilic addition of ethene and HBr?

• Br-

• carbocation

What is the final product of electrophilic addition of HBr and ethene?

Bromoethane

What happens when an unsymmetrical molecule undergoes an addition reaction with an alkene?

2 products are formed

What 2 products are formed when propene and hydrogen bromide react?

• 2 bromopropane

• 1 bromopropane

What can we label the 2 products as when 2 products are made from electrophilic substitution of alkene?

major and minor

How do we determine which is the major or minor product?

based on carbocation

What is Markownikoff’s rule?

when a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attatches itself to the carbon atom wth the larger number of hydrogen atoms

Are carbocations wit more alkyl groups more or less stable?

more

Why are carbocations with more alkyl groups attatched more stable?

the alkyl groups donate electrons toward the positive charge which allows the charge to spread

How are carbocations classified?

• primary (1 alkyl)

• secondary (2 alkyl)

• tertiary (3 alkyl)

Which is the major product?

the product in which the O or halide attatches to the C with the largest number of alky groups attatched

Why is the product where the more electronegative atom of unsymetrical molecule is attatched to carbocation with more alkyl groups major?

it is more stable

When alkenes join together what do they form?

addition polymers

What type of reaction is the joingin of many alkene monomers?

addition polymerisation

What are the reagents for addition polymerisation?

alkene monomer

What are the conditions for addittion polymerisation?

• high temperature

• high pressure

Draw the general equation for the formation of a polymer from alkene monomers

What does n represent in polymerisation?

thousands of monomer units

What is ethene polymerised to form?

poly(ethene)

What is polyethene used for?

packaging and bags

Give 2 problems with the disposal of addition polymers

• they are not biodegradeable

• burning produces toxic gases

Why are addition polymers non-biodegradable?

• the C chain is non polar and can not be broken down by hydrolysis

• they are very table and do not break down naturally or by microorhamisms at landfil sites

What can addition polymers produce when burned?

toxic gases

What gases are produced when pVC or other chlorine containing polymes is burnt?

HCl and Cl2

How can waste addition polymers be processed?

• combustion for energy production

• removal of toxic wste products

• used as organic feedstock

• sorted into types and recycled

What are the benefits of combusting addition polymers?

burned as fuel to produce energy

Give an example of removing toxic waste products from addition polymers?

removal of HCl formed during combustion of PVC

How are waste addition polymers used as organic feedstock for plastics?

they are cracked into smaler chain alkanes and alkenes as an organic feedstock to produce new plastics

What are the benefits of using waste addition polymers as organic feedstock for plastics?

feedstock recycling

What are the roles of chemists in minimising damage of addition polymers?

• develop biodegradeable poymers

• develop photodegradeable polym ers

• use an alkaline scrubbers to neutralise toxic HCl gas produced in burning

How are biodegradeable polymers broken down?

by microorganisms in water

Give an example of a biodegradeable polymer

PLA (polylacticacid)

What are photodegradeable polymers

polymers that are oil based that contain bonds weakened by absorbing light (C=O bonds)

How can we neutralise toxic HCl gas produced when chloropolymers are burned>

use an akaline scrubber