Alkenes.

Question 1

What type of hydrocarbon are alkenes.

Answer 1

They are unsaturated hydrocarbons. They will have at least one C=C somewhere within their structure.

Question 2

What is the general formula of an alkene.

Answer 2

General formula is CnH2n

Question 3

One is the one thing an alkene must contain in its structure.

Answer 3

Alkenes contain a carbon- carbon double bond somewhere in their structure.

Question 4

Describe , geometrically , the arrangement of bonds around the C=C.

Answer 4

the arrangement of bonds around the
>C=C< is planar and has the bond angle 120 degrees.
Key Points: Planar and 120 degrees.

Question 5

Draw and label the C=C double bond itself and hence explain the type of species this leaves alkene vulnerable to attack from as a result of this structure.

Answer 5

Should see picture on notes.
Description: C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond.

C-C Pi bond goes horizontal and is above and below the sigma bond which is sandwidged.

The carbon carbon sigma bond goes in between and sticks out vertically , two between each carbon.

Question 6

Define unsaturated.

Answer 6

Refer to “the presence off at least one C=C within the structure”.

Question 7

Defne electrophile.

Answer 7

An electrophile is a an electron pair acceptor, it likes the electrons so takes em.

Question 8

Give the change in functional group when bromine reacts with ethene , give the conditions and state also name and outline a mechanism for this reaction. Name the product that is formed.

Answer 8

Change in functional group: alkene INTO dihalogenoalkane

Reagent: Bromine
Conditions: Room temperature (not in UV light) Mechanism: Electrophilic Addition
Type of reagent: Electrophile, Br+ Plus is a dipole .
Product formed is 1,2-dibromoethane.

Question 9

Give the electrophile for when bromine molecule reacts with ethene. Draw also the intermediate which is formed and state the specific name for this structure.
Give the electrophile when hydrogen bromide reacts with ethene. Name and outline an overall mechanisms for this reaction.

Answer 9

(Brδ+)

Carbocation will form which is an intermediate species with a. positive charge.

Question 10

How can a bromine molecule Br2 undergo a electrophilic addition when it is a molecule that his not in any way polar due to the even distribution of electron density.
Give the practical application of this reaction.

Answer 10

As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+)

Question 11

Draw in displayed formula the reaction that takes place between HBr and But-2-ene. Name the product that is formed. State what is meant by symmetrical.

Answer 11

Product formed is 2-bromobutane. The alkene is symmetrical so swapping over the Br and H does nothing.
Remember symmetry is in regards to the double bond in the alkene not the number of carbon atoms that are present. If the double bond is dead in the middle of the chain then swapping over the H and the Br changes nothing about the structure as you can just flip it.

Question 12

Name and outline a mechanism for the reaction of hydrogen bromide and But-2-ene.

Answer 12

Will be electrophilic addition. That will form 2-bromobutane with no possible isomers.

Question 13

In what type of electrophilic addition reaction would there be two possible products and why. Also state why one particular reaction could not possibly have any isometric products form. State the type of isomerism in question.

Answer 13

If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products. The carbon carbon double bond needs to not be in the dead middle of the chain ( remember its not about charge length but more if the double bond is in the middle).
The reaction of Bromine molecule (not HBr) with any alkene , symmetrical C=C position or nor will not produce position isomers since shaping a bromine with bromine changes nothing.
Must be H-Br with an asymmetric alkene to produce two functional group isomers.

Question 14

Give Markownikoffs rule.

Answer 14

The major product will form via the negative species ( usually bromine but also with the sulphuric acid one ) adding to the most stable carbocation.
This means that in rather unhelpful terms the hydrogen will add to the carbon that has the most hydrogens already on it.
Just think the carbocation carbon that gets attached by Br: is the one with more alkyl groups on it.
Remember the Br-: that gets generated is not a electrophile that is the partially positive atom in the H-B
r . If anything this is a nucleophile.

Question 15

What is the one type of electrophilic addition reaction that will produce position isomers.

Answer 15

H-Br or H-Cl ( any hydrogen halide) with an asymmetric alkene. Not Br-Br.

Question 16

For the reaction of Hydrogen bromide with But-1-ene give the two possible reaction mechanism and state and explain the minor and major product of the compound naming each and giving their rough percentage within a mixture of product from this reaction. State why this reaction doesn’t produce a perfect mixture of the same proportion of each product.

Answer 16

On NOTES

Question 17

What are addition polymers.

Answer 17

Polymers are made from monomers that are based on ethene.

Question 18

Draw a reaction for propene becoming poly(propene).

Answer 18

Should be a C=C with a CH3 group coming of one of the carbons in an E/Z like form. ( Remember you name polymers based on the longest chain). No bracket for reactant. n comes before reactant.

In the products the n is after. Add in square brackets and the n comes after. Double bond breaks and goes through the brackets.

Question 19

How do you name polymers.

Answer 19

Poly(original alkene) Not the original alkane.

longest chain where the carbon double bond are the two central carbons.

Question 20

Draw the one repeating unit for poly(propene).

Answer 20

Square bracketed up Carbon SINGLE BOND carbon with one methyl group coming off any carbon. Bonds extend through the bracket. No “n”.

Question 21

Why are poly(alkene)s are unreactive and state also another homologous series that this is similar to.

Answer 21

Poly(alkenes) are unreactive due to the strong C-C and C-H bonds. They are also non polar.
This is similar to in alkanes.

Question 22

When and where should you include the famous 'n’ in regards to alkenes.

Answer 22

You need to include “n” when considering the reaction from a monomer into a large number of that polymer and never when asked to give one repeating unit. “n” is only for large numbers and this occurs in the polymerisation of an alkene monomer.
For the reaction then “n” goes before the monomer and after the polymer. This represents a large number of monomer gain g into a large number of repeating unit ( its not a single repeating unit so n is needed for polymerisation reactions.

Never for repeating unit.

Question 23

Give a general equation using R for the in which a monomer of alkene becomes a poly(alkene)

Answer 23

No brackets with R coming of of a carbon. “n” should be BEFORE the molecule with its double bond still in tact and no brackets.
This goes into Bracketed up molecule with the C=C now a C-C and outstretched reaching through the square brackets you have now put in.
The Bonds should stretch out through the square brackets.
There should be an “n” after the bracketed up product you have formed.

Question 24

What bond should be found in a repeating unit.

Answer 24

It is a C-C single bond in the repeating unit since the bond has already broken to form the polymer and so it would make to sense to have it as a C=C would it.

Question 25

How do you name polymers.

Answer 25

Based on the longest chain. If given a repeating unit it would be poly(longest chain ene).

Based on alkene.

Question 26

Draw the polymer repeating unit for But-2-ene.

Answer 26

Bracketed up Carbon SINGLE BOND carbon that has outstretched bonds through the square brackets. NO “n” its a single repeating unit.

methyl groups off of 2 carbons.

Question 27

Why are poly(alkenes) non biodegradable.

Answer 27

Saturated alkane backbone has strong and non polar C-H and C-C. This means it is not vulnerable to attack from biological agents like enzymes.

Question 28

Why are tertiary Carbocations more stable than primary.

Answer 28

Alkyl groups have a positive inductive affect so release electrons and stabilise the charge on the carbocation.

Question 29

What polymer is generally though of to have differing properties in regards to its high and low density form and which polymer is thought of in regards to its plasticised and unplasticised form.

Answer 29

• High and low density is Poly(ethene).
• Poly(chloroethene) (PVC) is considered in regards to varying properties from practicers.

Think PVC is more common in white gold and so is plactisicers rather than all that high and low density stuff.

Question 30

Explain feedstock recycling. And give a drawback.

Answer 30

Heat the plastic to break the polymer bonds into monomers , use to make new plastics.
Upon heating the chain length will become shorter as they break and hence the properties of the plastic degrade.
Think Feedstock is furnace.

Question 31

Explain mechanical recycling.

Answer 31

Separate the plastic wash it and remould it by grounding into pellets.
Think Mechanical is more simple style of writing an English essay.

Question 32

Give the conditions for producing low density poly(ethene) and state also the properties it results in this polymer having.
Do the same for high density.
Give some uses of each.

Answer 32

Both made by polymerising ethene but at different temps.
For low density : high temps and pressures.
Via Free radical mechanism creates branching so you get electrical cables as use since more flexible.

For high density: Lower temp and pressure (a little above room) with a Zeiger nata catalyst and you get less branching and more rigid.
Hence can be used for buckets.

Question 33

Give a use for poly(ethene) don’t worry about high or low density in your answer.

Answer 33

Plastic bags is easiest one.

Question 34

Give the use of poly(propene). Draw a repeating unit.

Answer 34

Car bumpers. C-C extended outside brackets with no “n”. Contains a methyl group on a carbon .

Question 35

Give , in structural formula , the reaction , of concentrated sulphuric acid with ethene. State the change in functional group.

Answer 35

CH2=CH2 + H2SO4 —> CH3CH2OSO2OH
alkene —-> alkyl hydrogensulfate
In this case ethyl hydrogen sulphate.

Question 36

State the names of the two states that would be done to produce an alcohol from a alkene. This is a different method than by hydration of ethene using the H+ from a. strong phosphoric acid catalyst. (That is from the alcohol topic)
Hint : Stage one is a mechanism
Stage two is just a normal reaction.

Answer 36

Stage 1: Mechanism: Electrophilic Addition

Stage 2:Type of reaction: hydrolysis

Question 37

State how the industrial production of ethene by straight hydration the alkene (in alcohol topic) differs from the method used with sulphuric acid in the alkene topic.

Answer 37

The use of sulphuric acid is a two step reaction where , sulphuric acid acts as a catalyst. It is split into two steps whereby in the electrophilic addition reaction you get an an alkyl hydrogen sulphate. This will have a polar bond with the C-O.
Water is then used in a nucleophilic substitution reaction where the water acts as a nucleophile and attacks the partially positive carbon and like the ammonia nucleophilic substitution, hydrogen needs to neutralise the charge on the electronegative atom , the hydrogen combines with the lost HSO4 and hence you get an alcohol from the OH left from the water and the sulphuric acid regenerated.
Water simply hydrates ethene at high temperatures and pressures in industry. With a phosphoric acid catalyst.

The Atom economy is better for the hydration of ethene since there is only one product rather than the XS reproduced H2SO4.
The separation process is easier and cheeper also.

Question 38

Give the conditions and balanced equation for the hydration of ethene in industry. State why this can’t be done in the laboratory.

Answer 38

CH2=CH2 + H20 —> CH2CH2OH Needs a concentrated phosphoric acid catalyst to provide the H+ that will start the whole thing off. Done at 300 degrees Celsius and 70atm.
Hence the high temps and pressures mean it can’t be done in the lab.

Question 39

Draw a repeating unit of poly(chloroethene).
State a use for it in its plasticised and unpracticed form.
Give some general properties of PVC that means it can do this.

Answer 39

Unplasticiced can be used for drainpipes.
Plasticised it can be used for electrical wiring.
PVC is Water proof and an electrical insulator.