2.5 - Hydrocarbons Flashcards by Luca Hodkinson

What is a fossil fuel?

Fuels that were derived from organisms that lived several million years ago

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What are the advantages of fossil fuels?

Available in many forms
Available at all times
Current infrastructure is generally set up around the use of fossil fuels

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What are the disadvantages of fossil fuels?

Non-renewable (take millions of years to form and are being used up very quickly)
CO2 is formed (greenhouse gas) that acts by absorbing infrared radiation from earth’s surface and the emitting it in all directions. Some of this radiation goes back towards Earth’s surface (increasing surface temp)
Acid rain (rain with a lower than expected pH)
Carbon monoxide formation

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How is acid rain formed?

Formed from the oxides of sulfur and nitrogen that are by-products of burning fossil fuels that contain sulfur and nitrogen

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When fossil fuels are burned in an adequate supply of oxygen, complete combustion occurs and carbon dioxide and water are formed. What happens if there is a shortage of oxygen?

Incomplete combustion occurs and carbon monoxide is formed. CO is toxic since it combines with haemoglobin in the blood so this is then not available to carry oxygen around the body.

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Alkanes
General formula?
How does each member in series differ?
Chemical properties?
Physical properties?

C(n)H(2n+2)
By CH2
Similar as they are saturated hydrocarbons
Vary as mass increases

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How does mass/size effect the state of an alkane?

Small alkanes are gases (methane)
larger ones are liquids (petrol)
Largest ones are solids (candle wax)

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Why are alkanes non reactive?

They are non polar as the electronegativities of C and H are similar
Have no multiple bonds, only single (sigma) bonds

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What is a sigma (σ) bond?

Made by end to end overlap of s or p-orbitals

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What 2 important reactions do alkanes undergo?

Combustion and halogenation

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Alkanes are used as fuels because they burn well in oxygen and produce energy in exothermic reactions. What is complete and incomplete combustion?

Complete - combustion that occurs with excess oxygen
Incomplete -combustion that occurs with insufficient oxygen

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What is halogenation?

The reaction between an organic compound and any halogen (member of group)

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Alkanes do not react with halogens in the dark. How do you get them to react?

when exposed to UV light

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What happens in the initiation stage of photochlorination of methane (radical substitution)?

UV light has sufficient energy to break Cl-Cl bond homiletically. Homolytic bond fission occurs - each of the boned atoms receives 1 of the bond electrons so that 2 radicals are formed.

See NC 1

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What is a radical?

A species with an unpaired electron

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What happens in the propagation stage of photochlorination of methane (radical substitution)?

Radicals are very reactive and take part in a series of propagation reactions. A molecule reacts with a radical to make a new molecule and a new radical. The reaction continues and so is a chain reaction.

See NC 1

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What happens in the termination stage of photochlorination of methane (radical substitution)?

The reaction that ends the process. Two radicals react to make a molecule

See NC 1

Alkenes
General formula?
Functional group?
Reactivity and why?
Formed?
Used as fuels?
Starting materials for…

C(n)H(2n)
C=C
Because of the double bond, they’re known as unsaturated hydrocarbons and are much more reactive than alkanes.
When long hydrocarbons are cracked
They will undergo combustion but are not generally used as fuels
organic synthesis reactions like polymerisation

Alkenes are ________ hydrocarbons meaning they contain a carbon-carbon double bond. This is an area of ___ ______ ______ making it susceptible to attack from ____________ (species that are attracted to negatively charged areas). The double bond consists of a normal σ-bond and a π-bond that wraps around the centre of the C=C bond. The π-bonds involve ________ overlap of p-orbitals above and below the plane of the molecule

unsaturated
high electron density
electrophiles
sideways

See NC 2

Single bonds in alkanes allow the atoms/groups at either end to rotate freely but the double bond in alkenes means that rotation is restricted. Why?

Due to the π bond

See NC 2

What is the difference between a Z and an E isomer?

An Z isomer has the 2 groups of higher priority (greater atomic mass) on the same side of the double bond
A E isomer has the 2 groups of high priority on opposite sides of the double bond

Melting temp and boiling temp depend on the strength of intermolecular forces. The strength of these forces is affected by how well the molecules pack together because this alters the area of contact between molecules. Does a Z or an E isomer have a higher melting temp and why?

E because In general the shape of E molecules means that they can fit together more closely. This means that they have stronger intermolecular forces and therefore higher melting temps

What is an electrophile?

An electron deficient species that can accept a lone pair of electrons

What is the difference between homolytic bond fission and heterolytic bond fission?

Homolytic fission - the splitting of a covalent bond where each atom retains one electron from the bonding pair
Heterolytic fission - the splitting of a covalent bond where one atom retains both electrons from the bonding pair

What does heterolytic and homolytic bond fission produce?

Hetero - oppositely charged ions Homo - free radicals

By undergoing electrophilic substitution an electrophile can form... from alkenes

alcohols or halogenoalkanes

Alkenes have a pair of electrons in a _ orbital that makes them susceptible to attack by an __________, and so alkenes are far more reactive than _______. An electrophile is any species that can accept a ____ ____ of electrons. The mechanism of the reaction involves ________ bond fission leading to an addition reaction. What is an addition reaction?

π electrophile alkanes lone pair heterolytic 2 molecules combine to give only 1 product See NC 3

What happens if the alkene is unsymmetrical?

Two halogenoalkane products are formed See NC 3

What is an alkyl group

An alkane missing 1 hydrogen

Carbocations have varying stability. Stability increases from primary to tertiary so that order of increasing stability we have... The more stable the carbocation, the more likely it is to form. Therefore in addition reactions, multiple products can form but the major product will always form from the most stable carbocation

Primary carbocation

What is the test for the presence of alkenes?

Bromine water (orange/brown) decolourises in the presence of alkenes. Due to addition reaction shown (electrophilic substitution) where brown Br is added to alkene and therefore is no longer bromine.

Alkenes can also react with hydrogen with the presence of a nickel catalyst. Why is this hydrogenation reaction important in food production?

As it converts liquid unsaturated oils into solid, more edible saturated fats. These are used as butter substitutes and as spreads (Electrophilic substitution)

What is polymerisation? What is addition polymerisation?

The joining of a huge number of monomer molecules to make a large polymer molecule. Addition polymerisation is the process of repeated addition of monomers that possess double or triple bonds to form polymers.

What are monomers?

A small molecule that can be made into a polymer. Alkenes are good examples of monomers as they undergo addition polymerisation where the double bond is used to join the monomers together. Nothing is eliminated in this process

Ethene is polymerised to form poly(ethene) What does the n represent? Does the polymer contain a double bond? The regions inside the brackets is called the...

The large number of monomers that are joined together in the polymer No, lines stick out from brackets repeat unit of the polymer

How do you name polymers?

Prefix poly- is added to the name of the alkene

Polyethene is used to make _____ ____ etc as its flexible and _________. When it was first made it had side branches which meant that the chains couldn't pack together tightly reducing the number of ___ ___ _____ forces, meaning their melting point was also low. Catalyst were developed that make polyethene into straight chains. This gives a polyethene with higher ______ and higher melting point. They are used where more rigidity is required and/or the temp of use is higher.

plastic bags unreactive Van der Waals density See NC 5

Different polymers can be made using substituted alkenes as the monomer. This makes polymers with a large variety of uses. Name some:

Polypropene Polychloroethene Polyphenylethene

What is the monomer, physical properties and use's of polypropene?

Monomer - propene It's rigid and used in food containers and kitchen equipment

What is the monomer, physical properties and use's of Polychloroethene (PVC)?

Monomer - Chloroethene Used as flexible waterproof covering for pipes or insulation covering of electrical cables

What is the monomer, physical properties and use's of Polyphenylethene (polystyrene)?

Monomer - phenylethane Hard and used in many household items needing strength and rigidity. Expanded polystyrene is a good insulator