Alkenes and Alcohols

Question 1

What is the general formula for alkenes?

Answer 1

CnH2n

Question 2

What is the key feature that separates alkenes from alkanes?

Answer 2

Alkenes have at least one carbon-carbon double bond

Question 3

Why are alkenes considered unsaturated?

Answer 3

They contain a double bond

Question 4

What are electrophilic addition reactions?

Answer 4

It is where the double bonds open up in alkenes and two atoms are added to the carbon atoms

Question 5

What are electrophiles?

Answer 5

They are electron pair acceptors. As they are usually short of electrons they tend to be attracted to areas where there are a lot of them. Like polar molecules and positively charged ions.

Question 6

How can you test if a hydrocarbon is unsaturated?

Answer 6

Test using bromine water, if the bromine water goes colorless it is an alkene and if it doesn’t it is an alkane (unless UV light)
This is an example of electrophilic addition.

Question 7

What is a carbocation?

Answer 7

A carbocation is an organic ion containing a positively charged carbon atom

Question 8

How are halogenalkanes formed using Alkenes?

Answer 8

Electrophilic addition. e.g. Adding HBr or Br-Br

Question 9

What happens when we add hydrogen halides to unsymmetrical alkenes?

Answer 9

Two possible products are formed- the amount of each is determined by how stable the carbocation formed in the middle of the reaction is- this is known as the carbocation intermediate

Question 10

How do we know what carbocation is the most stable?

Answer 10

Carbocations with the most alkyl groups are more stable because the alkyl group feeds electrons toward the positive charge. The more stable carbocation is much more likely to form.
Primary are the weakest then secondary and tertiary are the strongest
(These cause the formation of major and minor products)

Question 11

What will alkenes form when reacted with cold concentrated sulphuric acid?

Answer 11

They form Alkyl hydrogen sulfates. Which can then be converted into alcohol by adding water and warming the reaction mixture. (electrophilic addition reactions) The sulphuric acid is a catalyst and isn’t used up.

Question 12

What are monomers and polymers?

Answer 12

Polymers are long chain molecules formed when lots of small molecules, called monomers join together.

Question 13

What are the two types of polymers?

Answer 13

Natural: DNA and proteins
Synthetic: Polyethene

Question 14

What are addition polymers and how are they formed?

Answer 14

Alkenes can act as monomers and form polymers because their double bonds can open up and join together to make long chains. These types of polymers are called addition polymers.

Question 15

What are the properties of polyalkane chains?

Answer 15

Polyalkene chains are saturated molecules. The main carbon chain is also non-polar. These factors result in addition polymers being very unreactive.

Question 16

How do intermolecular forces affect the properties of polyalkenes?

Answer 16

Polyalkene chains are non-polar so the chains are only held together by van der Waals forces. The longer the polymer chains and the closer together they can get the stronger the van der Waals forces between them are (branched not as close). Therefore long, straight chains tend to be stronger and more rigid than short, branched chains, which tend to be weaker and more flexible.

Question 17

What are plasticizers and how do they work?

Answer 17

Adding a plasticizer to a polymer makes it more flexible. The plasticizer molecules get between the polymer chains and push them apart. This reduces the strength of the intermolecular forces between the chains- so they can slide around more, making the polymer easier to bend.

Question 18

What are the two forms of PVC?

Answer 18

You can have both rigid PVC to make drainpipes while plasticized PVC is used in electrical cable insulation and clothing.

Question 19

What are repeating units?

Answer 19

Polymers are made up of repeating units. The repeating units of addition polymers look very similar to the monomer, but with the double bond opened out.

Question 20

What is the functional group for alcohol?

Answer 20

-OH (hydroxyl group)

Question 21

How can you tell whether an alcohol a is primary, secondary, or tertiary alcohol?

Answer 21

It depends on what carbon atom the OH is bonded to
Primary= carbon on end (C only bonded to one other)
Secondary= carbon in middle (C bonded to two others)
Tertiary= carbon opposite chain (C bonded to 3 others)

Question 22

How can you make alkenes from alcohols, and why is it important?

Answer 22

You can make an alkene by eliminating water in a dehydrogenation reaction. This allows you to produce alkenes from renewable resources. Which in turn allows you to produce polymers without needing oil. One of the biggest industrial uses of alkenes is as a starting material for polymers.

Question 23

How do you dehydrogenate ethanol to form ethene?

Answer 23

1. Ethanol is heated with a concentrated sulphuric acid (phosphoric acid could also be used)
2. The product is usually in a mixture with water, acid, and reactant in it, so the alkene has to be separated.

Question 24

Why can dehydrogenation of more complicated alcohols have more than one possible product?

Answer 24

The double bond can form on either side of the carbon with the -OH group attached

Question 25

Why do we need distillation?

Answer 25

The products of organic reactions are often impure. Distillation is a technique that uses the fact that different chemicals have different boiling points to separate them. Often further separation and purification are needed though.

Question 26

How would you produce Cyclohexene from Cyclohexanol?

Answer 26

Distillation
- Add concentrated phosphoric and sulphuric acid to a round-bottomed flask with cyclohexanol
- add carborundum boiling chips (makes it boil more calmly)
- heated to around 83 degrees using a water bath
- condense gases produce and collect
Separation
- transfer the product mixture to a separating funnel and add water to dissolve water-soluble impurities and create an aqueous solution
- allow the mixture to settle into layers. Drain the aqueous lower layer, leaving the impure cyclohexane.
Purification
- Drain the cyclohexane into a round-bottomed flask
- Add anhydrous CaCl2 (drying agent) and stopper the flask
- Let the mixture dry for at least 20 minutes with occasional swirling
- Distil the mixture one last time

Question 27

How are alcohols produced from alkenes?

Answer 27

Alcohols are produced by the hydration of alkenes.
The standard industrial method for producing alcohols is to hydrate an alkene using steam in the presence of an acid catalyst.

Question 28

How can ethanol be produced from ethene?

Answer 28

Ethanol can be produced from the hydration of ethene by steam at 300 degrees Celsius and a pressure of 60 atm. It needs a phosphoric acid catalyst.

Question 29

Why is Fermentation to produce ethene better for the environment than hydration of ethene?

Answer 29

The ethene used in the hydration comes from cracking heavy fractions of crude oil. Crude oil is a finite resource that at this rate will eventually run out or become super expensive to use.

Question 30

How does the industrial production of ethanol by fermentation of glucose work?

Answer 30

• Fermentation is an exothermic process, carried out by yeast in anaerobic conditions around 30-40 degrees.
• yeast produces an enzyme that converts sugars, such as glucose into ethanol and carbon dioxide
• Once formed ethanol is separated from the rest of the mixture via fractional distillation
• needs to be the right temperature for the reaction to be fast enough but also not too hot as enzymes will denature
• Fermentation is low-tech and uses cheap materials and equipment. However, the extra fractional distillation step that is needed to purify the ethanol produced using this method takes extra time and money

Question 31

What is a biofuel?

Answer 31

Biofuel is a fuel that’s made from biological material that’s recently died.

Question 32

What is bioethanol?

Answer 32

Ethanol is increasingly being used as a fuel, particularly in countries with few oil reserves. For example, in Brazil, sugars from sugar cane are fermented to produce alcohol, which is added to petrol. Ethanol made in this way is a biofuel and is called bioethanol.

Question 33

What are the advantages/disadvantagesof biofuels?

Answer 33

• They are renewable energy sources, making them more sustainable
• They do produce CO2 when burnt, but it’s the CO2 the plant absorbed while growing so they are considered carbon neutral
• Petrol car engines would have to be modified to use fuels with high ethanol concentrations
• When you use land to grow crops for fuel, it can’t be used to grow food. Which could in turn lead to food scarcity

Question 34

Why do people argue that bioethanol is not carbon neutral?

Answer 34

While yes the moles of CO2 produced from fermentation and burning of ethanol are equal to the moles used in photosynthesis. However, fossil fuels will need to be burned to power the machinery used to make fertilizers for the crops, harvest the crops, and refine and transport the bioethanol. Burning the fuel to power this machinery produces carbon dioxide. So using bioethanol made by fermentation isn’t completely carbon neutral.

Question 35

What do you need to oxidize alcohols?

Answer 35

The oxidizing agent acidified potassium dichromate, to mildly oxidize alcohols. The orange dichromate ion is reduced to the green chromium ion.

Question 36

What are primary, secondary, and tertiary alcohols oxidized to?

Answer 36

Primary alcohols are oxidized to aldehydes and then to carboxylic acids.
Secondary alcohols are oxidized to ketones only
Tertiary alcohols aren’t oxidized

Question 37

What are aldehydes?

Answer 37

They have a hydrogen and one alkyl group attached to the carbonyl carbon atom. Their suffix is -al. You don’t have to say, which functional group is on- it is always on carbon 1

Question 38

What are ketones?

Answer 38

They have two alkyl groups attached to the carbonyl carbon atom. Their suffix is -one. For ketones with five or more carbons, you always have to say which carbon the functional group is on.

Question 39

What are carboxylic acids?

Answer 39

They have a COOH group at the end of their carbon chain. Their suffix is -oic acid.

Question 40

How can you control how far a primary alcohol is oxidized?

Answer 40

You can control the reaction conditions
- heating ethanol with potassium dichromate and sulphuric acid in a test tube produces ethanal (although it’s hard to stop it oxidizing to ethanoic acid)
- use a distillation apparatus and condenser to get just the aldehyde
- heat under reflux to get the carboxylic acid

Question 41

How does heating an alcohol under reflux produce a carboxylic acid?

Answer 41

Heating under reflux means that you can increase the temperature of an organic reaction without losing volatile solvents, reactants, or products. Any vaporized compounds cool, condense, and drip back into the reaction mixture. So the aldehyde stays in the reaction mixture and is oxidized to carboxylic acid.

Question 42

Can ketones be oxidized easily?

Answer 42

no

Question 43

Why can’t tertiary alcohols be oxidized easily?

Answer 43

Tertiary alcohols don’t react with potassium dichromate at all the solution stays orange. The only way to oxidize tertiary alcohols is by burning them.

Question 44

How can you distinguish between aldehydes and ketones?

Answer 44

They can be distinguished by oxidizing agents- aldehydes can be oxidized easily but ketones can’t.
- Fehling’s solution and Benedict’s solution are both deep blue Cu2+ complexes, which reduce to brick-red Cu2O when warmed with an aldehyde- but stay blue with a ketone.
- Tollen’s reagent is [Ag+(NH3)2]+ and it’s reduced to silver when warmed with an aldehyde, but not with a ketone. The silver with coat the inside of the apparatus to form a silver mirror.