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A Level Chemistry 4 > 4.1 - Basic Organic Concepts and Hydrocarbons > Flashcards

4.1 - Basic Organic Concepts and Hydrocarbons Flashcards

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1
Q

Define empirical formula.

A

The simplest whole number ratio of atoms of each element present in a compound. E.g. CH2.

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2
Q

Define molecular formula.

A

The actual number of atoms of each element in a molecule. E.g. C2H6.

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3
Q

Define general formula.

A

The simplest algebraic formula of a member of a homologous series. E.g. for an alkane: CnH2n+2.

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4
Q

Define structural formula.

A

The minimal detail that shows the arrangement of atoms in a molecule. E.g. butane would be CH3CH2CH2CH3 or CH3(CH2)2CH3.

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5
Q

Define displayed formula.

A

The relative positioning of atoms and the bonds between them.

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6
Q

Define skeletal formula.

A

The simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups.

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7
Q

Define homologous series.

A

A series of compounds having the same functional group but with each successive member differing by CH2.

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8
Q

Define functional group.

A

A group of atoms responsible for the characteristic reactions of a compound.

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9
Q

Define alkyl group.

A

A hydrocarbon branch from the main chain of an organic compound. Alkyl groups have the general formula CnH2n+1

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10
Q

Define aliphatic.

A

Compounds containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings.

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11
Q

Define alicyclic.

A

An aliphatic compound containing a non-aromatic ring.

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12
Q

What does aromatic mean?

A

Compounds that contain a benzene ring.

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13
Q

What does saturated mean?

A

Organic compounds containing only carbon-carbon single bonds.

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14
Q

What does unsaturated mean?

A

Organic compounds containing carbon-carbon double bonds, carbon-carbon triple bonds or aromatic groups.

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15
Q

Give the names of the first ten members of the alkanes homologous series.

A

Methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane.

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16
Q

Describe how to name compounds.

A

Number the principal chain from one end to give the lowest numbers.
Side chain names appear in alphabetical order - butyl, ethyl, methyl, propyl, etc.
Each side-chain is given its own number.
If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa, etc.
Numbers are separated from names by a hyphen, e.g. 2-methylheptane.
Numbers are separated from numbers by a comma, e.g. 2,3-dimethylbutane.
Alkenes - always number so the double bond is in the longest chain and number the chain so that the carbon in the double bond has the lowest number possible.

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17
Q

Give the definition for a structural isomer.

A

Compounds with the same molecular formula but different structural formula. E.g. C4H10 could be butane or 2-methylpropane.

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18
Q

What is the general formula for an alkane?

A

CnH2n+2.

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19
Q

Why are alkanes saturated?

A

They all contain carbon-carbon single bonds.

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20
Q

Describe the bonding in alkanes.

A

The carbon atoms in alkanes form sigma bonds by the overlap of atomic orbitals in a straight line. The atoms can rotate freely.

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21
Q

Describe and explain how the boiling point of alkanes vary with chain length.

A

Increased chain length = increased boiling point.

Longer chain has more electrons so greater van der Waals’ forces.

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22
Q

State the bond angle and shape formed around each carbon atom in alkanes.

A

Bond angle - 109.5°.
Shape - tetrahedral.
Bonds - Sigma (σ bonds).

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23
Q

Explain the bond angle, shape and type of bond formed in alkanes.

A

Electron pairs repel each other equally. There are four bonded pairs and no lone pairs.

24
Q

Explain how side chains affect the boiling point.

A

Branched chains have lower boiling points. Branches reduce the amount of surface contact between the molecules (the more side chains you have, the lower the boiling point).

25
Q

Explain why alkanes are not very reactive.

A

Non polarity of C-H and C-C bonds due to symmetrical shape. This makes it difficult to attract positively or negatively charged particles. The sigma bonds (C-C and C-H) are very strong with a large bond enthalpy. This makes them difficult to break.

26
Q

Describe the combustion reactions of alkanes.

A

Alkane + oxygen –> carbon dioxide + water.

Large amount of energy released leads to use as fuels in industry, in the home and in transport.

27
Q

Describe the incomplete combustion of alkanes.

A

Alkane + oxygen –> carbon monoxide + water.
This happens when there is a limited supply of oxygen.
Carbon monoxide is poisonous and prevents the transport of oxygen in the blood.
Faulty gas fires in poorly ventilated homes can cause deaths due to carbon monoxide poisoning.
Carbon monoxide can be produced from car exhausts and has to be regulated.

28
Q

Why is society so dependent on fossil fuels?

A

Energy (transport, electricity) and raw materials for numerous products (plastics).

29
Q

Why are we running out of fossil fuels?

A

Non renewable - used at a much faster rate than produced.

30
Q

Describe the substitution reaction of alkanes with chlorine or bromine.

A

CH3CH3(g) + Br2(l) –> CH3CH2Br(g) + HBr(g)

Condition: ultraviolet radiation.

31
Q

What is a radical?

A

A species with an unpaired electron.

32
Q

Describe the different types of covalent bond fission (breaking).

A

Homolytic fission. Each atom takes one electron from the covalent bond. Radicals are produced.
Heterlytic fission. One atom takes both of the electrons in a covalent bond. Produces a negative and a positive ion.

33
Q

What does a ‘curly arrow’ represent?

A

The movement of an electron pair. This can represent the formation or the breaking of a covalent bond.

34
Q

Describe the radical substitution mechanism involved in the reaction of alkanes with halogens. E.g. CH4 + Cl2 –> CH3Cl + HCl.

A

Initiation: homolytic fission of halogen molecule to form free radicals. Requires UV light. E.g. Cl-Cl(g) –> Cl•(g) + Cl•(g).
Propagation: CH4 + Cl• –> •CH3 + HCl. The Cl• will always take an H. •CH3 + Cl2 –> CH3Cl + Cl•. The organic radical always takes a halogen.
Termination: two free radicals react together. Cl• + Cl• –> Cl2. •CH3 + •CH3 –> C2H6. This explains why you can form larger organic molecules. •CH3 + •Cl –> CH3Cl.

35
Q

Explain why radical substitution reactions are not suitable for preparation of pure products.

A

Further substitution of hydrogen can occur producing more than one organic product.

36
Q

What is the general formula for alkenes?

A

CnH2n.

37
Q

Why are alkenes unsaturated?

A

They contain carbon-carbon double bonds.

38
Q

Describe the double bond in alkenes.

A

Double bond consists of 1 sigma bond, and a pi bond. The pi bond is formed by the sideways overlap of adjacent p orbitals.

39
Q

State the bond angle and shape formed around each carbon atom in alkenes.

A

Shape - trigonal planar.

Bond angle - 120°.

40
Q

Explain the bond angle and type of bond formed in alkenes.

A

Electron pairs repel each other equally. There are three bonded areas and no lone pairs.

41
Q

What is a stereoisomer?

A

Compounds with the same structural formula but with a different arrangement in space (e.g. E/Z isomerism).

42
Q

What is E/Z isomerism?

A

An example of stereoisomerism, arising from restricted rotation about a double bond. Two different groups must be attached to each carbon atom of the C=C group.

43
Q

What is cis/trans isomerism?

A

A special case of E/Z isomerism where two of the substituent groups are the same.

44
Q

How do you identify which is the E- isomer and which is the Z- isomer?

A

The E- isomer has the same group on opposite sides. Trans is the same group on opposite sides.
The Z- isomer has the same group on the same side. Cis is the same group on the same side.

45
Q

Why does E/Z isomerism occur?

A

The double bond cannot rotate. Each carbon atom of the double bond is bonded to two different groups.

46
Q

Describe the Cahn-Ingold-Prelog (CIP) priority rules to identify the E and Z stereoisomers.

A

Compare the atomic number of the two atoms directly attached to the carbon in the double bond.
The atom with the highest atomic number is given priority 1, the other is given priority 2.
Repeat the process for the other carbon in the double bond.
If it is a tie add the atomic numbers of the atoms directly attached to the atom that is attached to the carbon in the double bond.
Repeat the process if there is still a tie.

47
Q

Describe the relative reactivity of alkenes.

A

Alkenes are much more reactive than alkanes. The double bond has a high level of electron density. The pi bond sticks out above and below the molecule. This allows it to be attacked by electrophiles. The pi bond has a low bond enthalpy.

48
Q

What is an electrophile?

A

An electron pair acceptor.

49
Q

Describe the addition reactions of alkenes.

A

An alkene with hydrogen requires a nickel catalyst. CH2CH2 + H2 –> CH3CH3.
An alkene with a halogen. CH2CH2 + Br2 –> CH2BrCH2Br. Bromine is used as the test for a carbon double bond. The orange bromine water will decolourise if a double bond is present.
An alkene with hydrogen halides. CH2CH2(g) + HCl(aq) –> CH3CH2Cl(l). Asymmetrical alkenes (i.e. where the double bond is not in the middle of the molecule) will result in two isomeric products. CH3CHCH2 + HBr –> CH3CHBrCH3 or CH3CH2CH2Br.
An alkene with steam requires an acid catalyst (e.g. phosphoric). CH2CH2(g) + H2O(g) –> CH3CH2OH. Asymmetrical alkenes (i.e. where the double bond is not in the middle of the molecule) will result in two isomeric products.

50
Q

Explain the difference between a symmetrical and an asymmetrical alkene.

A

A symmetrical alkene will have the double bond in the centre of the chain, whilst an asymmetrical alkene will not.

51
Q

What is Markovnikov’s rule?

A

When a compound HX is added to an asymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.

52
Q

Why is it difficult to dispose of polymers?

A

Alkanes are very unreactive and break down very slowly. Non-biodegradable.

53
Q

Why is it important to process waste polymers?

A

If polymers are not processed they are added to landfill sites. These will cause environmental damage.

54
Q

Describe 3 methods of dealing with waste polymers.

A

Separation into types and recycling - but this is very costly.
Combustion to provide energy - pollutant gases can be removed via gas scrubbers. But toxic gases can be produced.
Feedstock recycling - polymers are cracked to produce alkanes and alkenes which can then be reused.

55
Q

Describe the benefits to the environment of development of biodegradable and photodegradable polymers.

A

Biodegradable polymers will decompose naturally. This removes the need to process them as waste and reduces the amount of environmental damage.

A Level Chemistry 4 (12 decks)

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