Alkenes ✅

need to do flashcards on polymers

are alkenes saturated or unsaturated

unsaturated

give the general formula for alkenes

C_nH_2n

for a carbon chain length longer than three carbons, what must you do

state the position of the double bond

e.g. but-1-ene vs but-2-ene

what type of bonds attach hydrogens to carbons in an alkene

sigma bonds

what how is a pi bond formed

• formed by the sideway overlap of adjacent p orbitals above and below the bonding C atoms

why does a pi bond mean that C=C double bonds can’t rotate

• overlap of 2 p-orbitals forms an orbital with a cloud of electron density above and below the single bond

• called a pi orbital

• meaning C=C bonds can’t rotate

a C=C is comprised of a … bond surrounded by a … bond

• sigma

• pi

what is the shape around carbon atoms in an alkene and what is the bond angle

trigonal planar

120 ( 3 bonding regions and 0 lone pairs )

define stereoisomerism

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

give another name for E/Z isomerism

geometric isomerism

read page 6 in alkenes booklet that talks you through E/Z isomerism and naming

okay

define electrophile

electron pair acceptor

what type of reactions do alkanes undergo and what type of reactions are these most commonly

• addition

• electrophilic addition

Addition of Hydrogen

1. name for this reaction

2. reagents

3. conditions

4. draw out the displayed equation for reaction of ethene + H₂

5. what happens in this reaction

6. when is it done- example

1. hydrogenation

2. H₂

3. Nickel catalyst

4. okay

5. unsaturated alkene becomes saturated

6. used in manufacture of margarine

Addition of halogens

1. name for this reaction

2. reagents

3. conditions

4. draw out the displayed equation for reaction of ethene + Br₂

5. when is addition of bromine carried out

6. when is addition of iodide carried out

1. halogenation

2. Cl₂ Br₂ I₂

3. room temp

4. okay

5. laboratory test for the presence of double bonds

6. to test for unsaturation in vegetable oils

describe the chemical test for alkenes

• add bromine water

• shake

• orange → colourless

Addition of hydrogen halides

1. name for this reaction

2. reagents

3. conditions

4. draw out the displayed equation for reaction of ethene + HCl

5. when is addition of bromine carried out

6. when is addition of iodide carried out

1. halogenation

2. HCl HBr or HI

3. room temp

order the first 4 hydrogen halides in order or decreasing bond enthalpy

H-F, H-Cl, H-Br, H-I

Reaction with Steam

1. name for this reaction

2. reagents

3. conditions

4. draw out the displayed equation for reaction of ethene + H₂O

5. when is this reaction carried out

1. hydration

2. H₂O _(g) (temp>100*C)

3. Phosphoric acid H₃PO₄ catalyst

4. okay

5. addition of steam across a double bond is used in industrial manufacture of alcohols

Reaction with concentrated H₂SO₄

1. reagents

2. conditions

3. draw out the displayed equation for reaction of ethene + H₂SO₄

4. name the product formed

5. what can the sulphuric acid be used as

6. what is the electrophile in this reaction

1. H₂SO₄

2. room temp

3. okay

4. ethylhydrogensulphate

5. a catalyst for the addition of water

6. partially positively charged H atom in sulphuric acid molecule

why do addition reactions happen with alkenes

pi bond is easily broken

so atoms can be added

what type of product is formed in an addition reaction

saturated

what happens when an unsymmetrical alkene goes under an addition reaction with hydrogen halides or other non symmetrical molecules

2 products are formed

• 1 minor and 1 major

the hydrogen becomes attached to the carbon with the mot hydrogen attached to it already

therefore that will produce the major product

draw the mechanism for the electrophilic addition reaction of ethene (symmetrical molecule) with a symmetrical molecule e.g. Br₂

okay

draw the mechanism for the electrophilic addition reaction of ethene (symmetrical molecule) with an unsymmetrical molecule e.g. HBr

okay

Unsymmetrical alkenes

1. what is formed when unsymmetrical alkenes undergo reactions with hydrogen halides and other non symmetrical molecules

2. why does this happen

3. e.g. draw the mechanisms with propene with hydrogen bromide

1. major and minor product is formed

2. major product is formed from carbocation with more alkyl groups attached which is therefore more stable carbocation ( always state classification of carbocation for major and minor products in exam )

remember: when drawing mechanism for major product, H attaches to the C either side of double bond which has the most H’s attached to it already!

What is a primary carbocation

Carbocation bonded to 1 C atom

What is a secondary carbocation

Carbocation bonded to 2 C atoms

What is a tertiary carbocation

Carbocation bonded to 3 C atoms

define addition polymerisation

reaction in which many monomers containing at least one C=C double bond form long chains of polymers as the only product

define term polymer

long chain molecule that is made up of many repeating units

steps for drawing repeat unit of polymer when given monomer

1. double bond → single bond

2. brackets and n

3. extending bonds either side out of brackets

yes

draw the displayed formula for the repeat unit of poly(ethene)

okay

draw the displayed formula for the repeat unit of poly(chloroethene)

okay

how does adding a plasticiser change a polymer

• makes it more flexible

• by preventing the polymer chains from being close to one another

• this disrupts the van der Waal forces between the chains, making them weaker

• so chains slide over each other easily

what can PVC be used for

1. before adding plasticiser

2. after adding plasticiser

1. making drainpipes

2. make pool liners

explain why poly(alkenes) are not biodegradable

• inert and very unreactive

• made up of strong C-C bonds

• no C=C double bonds

deduce the monomer of this addition polymer

how many carbons does a repeating unit have in the main carbon polymer chain

2

why is the disposal of polymers a challenge

due to their:

1. unreactivity

2. non-biodegradability

3. harmful combustion products when burnt

name 2 types of recycling to help reduce pollution by plastics

1. mechanical recycling

2. feedstock recycling

how does mechanical recycling act as a solution to pollution by plastics

• different types of plastic are separated and washed thoroughly

• ground up into small pellets

• pellets are heated until they melt, then remoulded so that they can be used again

• all without breaking down its polymer structure

• primary method for processing plastic waste

how does feedstock recycling act as a solution to pollution by plastics

• Heating the plastic to a high enough temperature that the polymer bonds break, and monomers are formed

• The monomers formed are then used to produce new plastics

describe an issue with these methods of recycling plastics

• Some plastics, like poly(propene), can only be heated and reused a number of times

• Each time the plastic is heated, some of the key chains break

• This means that over time, the properties of the plastic are degraded