A2 Structures, Reactions And Properties Of Commercially Important Organic Compounds

Question 1

General formula for alkanes

Answer 1

CnH2n+2

Question 2

Structural isomer

Answer 2

Have the same molecular formula but different structural formula.

Question 3

Chain isomer

Answer 3

Molecules with the same molecular formula but different arrangement of carbon atoms.

Question 4

Naming branched chain alkanes

Answer 4

1) find the longest carbon chain (name eg butane)
2) find any side chains and name (eg methyl)
3) number the side chains
Eg 2-methylbutane

Question 5

Complete combustion

Answer 5

• short chain alkanes are valuable fuels
• when there is plentiful supply of oxygen it undergoes complete combustion to form carbon dioxide and water.

Question 6

Uses of Methane and it’s equation for complete combustion

Answer 6

• main constituent of natural gas used for heating and cooking
• CH4 + 2O2 = CO2 + 2H2O

Question 7

Incomplete combustion

Answer 7

When there is a limited amount of oxygenation carbon monoxide gas, or carbon and water is formed.

Question 8

Incomplete combustion of methane

Answer 8

• CH4 + 3/2 o2 = CO + 2H2O
• CH4 + O2 = C + 2H2O

Question 9

What make’s intermolecular forces stronger (in alkanes) ?

Answer 9

• alkanes have induced dipole dipole forces
• these get stronger when there are more electrons
• larger alkane- stronger intermolecular force

Question 10

How can a change of shape affect the boiling point of alkanes?

Answer 10

• when the surface area of the molecule decreases but the number of electrons stay the same the boiling point decreases.
• this is because there is less surface contact and weaker intermolecular force.

Question 11

Formula for alkenes

Answer 11

CnH2n

Question 12

Positional isomerism

Answer 12

Molecules with the same molecular formula which have the functional group on different positions in the molecule.

Question 13

Functional group isomerism

Answer 13

Molecules with the same molecular formula but different functional groups.

• cycloalkanes and alkenes can show functional isomerism. Cycloalkanes have the same formula as alkenes CnH2n

Question 14

Stereoisomer

Answer 14

molecules with the same structural formula but a different arrangement of the atoms in space eg but-2-ene CH3CH = CHCH3

Question 15

Why do stereoisomers occur?

Answer 15

• double bonds restrict rotation
• when 2 different groups are attached to the carbons in the double bond

Question 16

Sp3 hybridisation - methane, CH4

Answer 16

• the s orbitals increases in energy
• the p orbitals decrease in energy
• all orbitals contain 1 electron
• the hybrid orbitals all have the same size and shape.
• 1s and 3p orbitals combine
• this makes 4 sp3 hybrid orbitals
• there are no spare p orbitals

Question 17

What bonds do sp3 hybridisation form?

Answer 17

-All hybrid orbitals form sigma bonds (single bonds)
- they do not form double bonds as there are no electrons in p orbitals

Question 18

Sp3 bond angle

Answer 18

109.5 degrees

Question 19

Sp2 hybridisation- ethene C2H4

Answer 19

• the s orbital increases in energy
• the p orbital decreases in energy
• all orbitals contain one electron
• the hybrid orbital all have the same size and shape.
• 1s and 2p orbitals combine
• this makes 3 sp2 hybrid orbitals
• there is one spare p orbital

Question 20

What binds do sp2 hybridisation make?

Answer 20

As there are 3 hybrid orbitals they form sigma bonds. There are spare p orbital electrons on each carbon which overlap to form pi bond which makes the double bond.

Question 21

Sp hybridisation- ethyne C2H2

Answer 21

• s orbital increases in energy
• p orbital decrease in energy
• all orbitals contain 1 electron
• the hybrid orbital all have the same shape and size
• 1s and 1p orbitals combine
• this makes 2 sp hybrid orbitals
• there are 2 spare p orbitals

Question 22

CH4 + cl2 = CH3Cl + HCl Free radical substitution mechanism

Answer 22

Initiation
- A free radical contains a single unpaired electron.

• Uv light provides the energy needed to break the cl-cl bond (halogen-halogen)
• both cl have donated one pair of electrons into the covalent bind
• this is called homolytic fission
• cl2 = 2Cl•

Propagation
- Cl• + CH4 = HCl + •CH3
- •CH3 + Cl2 = CH3Cl + Cl•
The second propagation reaction forms the desired haloalkane product and generates another chloride radical which can react again and again.

Termination
- this is where the free radicals combine
- Cl• + Cl• = Cl2
- •CH3 + •CH3 = C2H6
- •CH3 + Cl• = CH3Cl

Question 23

Ethanol uses

Answer 23

• as an intermediate in the synthesis of other organic chemicals
• as a solvent in cosmetics and perfumes
• manufacture of drugs, detergents and inks
• alcohol found in alcoholic drinks

Question 24

Producing ethanol from crude oil - hydration

Answer 24

Ethene is made when crude oil fractions are cracked to form an alkanes and alkene. Ethene is then hydrated with water in the presence of a concentrated phosphoric catalyst.

Ch2=Ch2 + H2O = CH3Ch2OH

Conditions 450 degrees, 50-100atm pressure

Question 25

Producing ethanol by fermentation

Answer 25

- carbs in plants are converted into sugars which are then fermented into ethanol using enzymes from yeast. - C6H12O6 = 2CH3CH2OH + 2CO2 Conditions yeast enzymes , temp 35degrees, no oxygen Too cold and the rate of reaction is slow Too high temp enzymes can denature and stop working No oxygen should be present ethanol can oxidise to ethanoic acid .

Question 26

Hydration of Ethene - pros cons

Answer 26

Advantages - large scale production method High purity product made - readily available raw material - high rate of reaction - continuous product method Disadvantages - high temp and pressure needed which increase costs - expensive and high tech equipment needed - crude oil is non renewable

Question 27

Fermentation- pros cons

Answer 27

Advantages - used renewable resources - low tech method little expense needed - readily available and renewable raw materials from plant - low cost Disadvantages - slow rate of reaction - impure ethanol made needs distilling - causes food prices to rise if crops are used to produce alcohol instead of food - batch production method- less efficient

Question 28

Explain why oil companies need to crack “heavy fractions”

Answer 28

As smaller chain molecules are higher in demand

Question 29

State the type of cracking that produces a high percentage of alkenes and give 2 conditions

Answer 29

Thermal cracking Conditions - high temp and high pressure