15. Alkenes Flashcards
Which is more reactive? Alkenes or alkanes?
Alkenes are more reactive because they have a double bond
What type of hydrocarbons are alkenes?
They are unsaturated hydrocarbons
How are alkenes produced?
Alkenes are produced in large quantities when crude oil is thermally cracked
What is the general formula for alkenes?
CnH2n
Which is the simplest alkene?
Ethene
What type of molecule is ethene?
A planar molecule with bond angle 120 degrees
There is no rotation about the carbon to carbon double bond in alkenes. Why?
The formation of the pi orbital. This is restricted rotation.
Alkenes with more than how many carbons can form different types of isomers?
alkenes with more than three carbons.
What two types of isomers are there in alkenes?
Positional isomers
Geometrical isomers
What are geometrical isomers?
Same structural formula but bonds are differently arranged in space.
(Stereoisomerism)
What is a Z isomer?
When the chains with the greatest Ar are on the same side
What is an E isomer?
When the chains with the greatest Ar and on opposite sides.
What are the physical properties of alkenes?
The double bond does not effect the boiling point. van der Waals forces are the only intermolecular forces that act between the alkene molecules. They are not soluble in water as they are not polar.
How do alkenes react?
The double bond makes a big difference to the reactivity of alkenes as compared with alkanes. Alkenes are more reactive because although the double C-C bond has a high bond enthalpy, it forms an electron-rich area which can easily be attacked by positively charged reagents.
What are positively charged reagents?
These are called electrophiles. They are electron pair acceptors one example is the H+ ion.
What are most of the reactions of alkenes?
Electrophillic additions
Alkenes will burn in air to produce…
Carbine dioxide and water
What are alkenes not used as fuels?
Their reactivity makes them very useful for other purposes.
Describe the mechanism of an electrophilic addition
- The electrophile is attracted to the double bond
- Electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion or have a positively charged area
- A positive ion (a carbocations) is formed
- A negatively charged ion forms a bond with the carbocation.
How do the arrows work in electrophillic addition?
Two stages
- Arrow from double bond to H or Br with positive partial charge
- Arrow from hydrogen halide/halogen bond to negatively charged component
- Negatively charged component now is a nucelophile which attacks the positive carbon (one with most hydrogen around it)
There are alkyl groups which release electrons to stabilise carbocations what is this electron release called?
Positive inductive effect
How do some halogens become electrophiles?
A halogen molecules is, at any moment, likely to have an instantaneous dipole which is attracted to the electron-rich double bond in the alkene.
What is the product formed from an electrophillic addition with sulfuric acid as the electrophile?
Ethyl hydrogensulfate.
If you add water to ethyl hydrogensulfate, what do you make?
Ethanol and sulfuric acid (catalyst for the process)
What is the test for a double bond?
Few drops of bromine water (reddish-brown)
Added to an alkene
Solution turns colourless
Alkenes can…
Polymerise and form long chains
What is the repeating unit of a polymer?
The smallest group of atoms that produce the polymer when repeated over and over.
Turning the ethene monomer into polythene is called…
Addition polymerisation
Is a polymer made from alkene monomers an alkene or an alkane?
Despite ending in -ene, the polymer is actually an alkene and is therefore unreactive.
What is a problem with plastics?
They are not biodegradable. This fills up landfill sites. Animals can ingest it.
How do we solve the problem of plastics not being biodegradable?
Reduce the amount of plastic we use or recycle it.
Describe mechanical recycling
Plastics are collected, separated, washed and sorted then ground into small pellets. They are melted and remoulded.
Describe feedstock recycling.
Plastics are heated which breaks polymer bonds and produces monomers which can then be used to make new plastics.
(Can only be done a limited number of times because on heating some chains break and become shorter, degrading the plastics properties.
Low density polythene is made with…
High pressure and high temperature
Branched chains do not pack together very well
High density polythene is made with…
Low temperatures and pressures and a Ziegler-Natta catalyst.
Less branching. The chains can pack together well.
What is the functional group of alcohols?
-OH
What is the general formula of an alcohol?
CnH2n+1OH
Shortened to R-OH
Use di-, tri- and tetra- to say how many -OH groups there are and numbers to say where they are just before the -ol ending.
For example:
Butane-1,4-diol
How do you tell if an alcohol is primary, secondary or tertiary?
Depending on how many other carbons the carbon that the OH group is bonded to.
(No other Cs bonded to the C next to the OH group are counted as primary too).
Why do alcohols have higher melting and boiling points than alkanes of similar relative molecular mass?
The -OH group in alcohols means that hydrogen bonding occurs between the molecules.
(Hydrogen bonding also makes the shorter chain alcohols soluble in water because hydrogen bonds can form between the -OH groups and water molecules).
Why are alcohols important in the in industrial chemistry?
Because they are used as intermediates. They are easily made and easily converted into other compounds
What is the molecular formula for ethanol?
C2H5OH
Write the equation for making ethanol from crude oil
CH2=CH2 + H2O —> C2H5OH
Ethene Water Ethanol
What is the catalyst for making ethanol from crude oil?
Phosphoric acid
Write the equation for making ethanol by fermentation.
C6H12O6 (aq) —> 2C2H5OH (aq) + 2CO2 (g)
Glucose Ethanol Carbon dioxide
What are the conditions for fermentation?
About 35 degrees (compromise between rate of reaction and enzymes in yeast denaturing)
vacuum to prevent oxidation of ethanol to ethanoic acid
Once fermenting solution contains 15% ethanol the enzymes are unable to function and fermentation stops.
What is the: i) rate of reaction ii) type of process iii) purity Of making ethanol from crude oil?
i) fast
ii) continuous
iii) pure
What is the: i) rate of reaction ii) type of process iii) purity Of making ethanol by fermentation?
i) slow
ii) batch
iii) aqueous solution of ethanol is produced
What is ethanol often used for?
As motor fuel
What is ethanol made by fermentation often termed?
Carbon-neutral fuel.
Show how making ethanol by fermentation is carbon neutral using equations.
- Photosynthesis in a growing plant; carbon dioxide is absorbed
6H2O (l) + 6CO2 —> C6H12O6 (aq) + 6O2 (g) - Fermentation; carbon dioxide is released
C6H12O6 (aq) —> 2C2H5OH(aq) + 2CO2 (g) - Combustion
2C2H5OH(aq) + 6O2(g) —> 4CO2 (g) + 6H2O (l)
Alcohols burn completely in oxygen to make…
Carbon dioxide and water (carbon monoxide or carbon if incomplete)
What is another use of ethanol?
Usage as a fuel
Combustion is complete oxidation. Alcohols can also be…
Oxidised gently and in stages
Primary alcohols are oxidised to…
Aldehydes, which can be further oxidised to carboxylic acids
Give the equation for the oxidation of ethanol to ethanal
C2H5OH(l) + [O] —> C2H3HO(g) +H2O(l)
Alcohol in excess ethanal
No reflux
Give the equation for the oxidation of ethanol to ethanoic acid
C2H5OH(l) + 2[O] —> CH3COOH (g) +H2O (l)
Oxidising agent in excess ethanoic acid
reflux
What are secondary alcohols oxidised to?
Ketones
Ketones are not oxidised any further
Give the equation for the oxidation of a secondary alcohol
Alcohol + [O] —> ketone + H2O
What are tertiary alcohols oxidised to?
Tertiary alcohols are not easily oxidised because oxidation would need a C-C bond to break rather than a C-H bond.
What has pleasant smells?
Many aldehydes and ketones
What are the experimental details of oxidising ethanol to ethanal?
We use dilute acid and potassium dichromate (vi).
We heat the mixture gently in the apparatus where the ethanal is condensed as soon as it is formed (boiling temp - 294K) and placed in a beaker. (The receiver is cooled in ice)
This prevents it from being oxidised further to ethanoic acid.
What are the experimental details of oxidising ethanol to ethanoic acid?
We use concentrated sulfuric acid and excess potassium dichromate (vi). We reflux the mixture (vapour condenses and drips back into the reaction flask).
After refluxing for 20 minutes we can distil off the ethanoic acid (boiling point 391K by using the same set up as when oxidising ethanol to ethanal (without the ice).
When oxidising ethanol which ions are reduced?
The orange dichromate (vi) ions are reduced to green chromium (iii) ions.
When oxidising a secondary alcohol to a ketone we use…
Acidified dichromate (we do not need to worry about further oxidation) E.g reflux is not necessary.
What is the functional group for a carboxylic acid?
COOH
What is the functional group for an aldehyde?
It is C=O at the end of the hydrocarbon chain
What is the functional group for ketones?
C=O the functional group is in the body of the hydrocarbon chain.
What suffix do ketones have?
-one
What suffix do aldehydes have?
-al
What are the names of the two tests for aldehydes and ketones?
The Tolens’ test
The Fehling’s/Benedict’s test
How does the Tollens’ test work?
Tollens’ reagent is a gentle oxidising agent.
It is a solution of silver nitrate in aqueous ammonia.
It oxidises aldehydes but has no affect on ketones.
It contains colourless silver (I) complex ions which are reduced to metallic silver (when warmed) as the aldehyde is oxidised (mirroring)
How does the Fehling’s test work?
It is a gentle oxidising agent.
They contain blue copper (II) ions which will oxidise aldehydes but not ketones.
The blue solution gradually changes to a brick red precipitate of copper (I) oxide in the presence of an aldehyde.
What are the elimination reactions of alcohols always?
Dehydrations - the leaving group is the -OH group and a hydrogen atom from the carbon next to the -OH group.
Ow are alcohols dehydrated?
With excess hot concentrated sulphuric acid or by passing their vapours over heated aluminium oxide.
What are the products of the hydration of an alcohol?
An alkene and water molecule.
What is an alternative dehydrating agent?
Phosphoric (v) acid
What is the general formula for an aldehyde?
CnH2n+1CHO
n= carbons excluding functional group carbon
