Topic 6 organic chemistry part 1

Question 1

What are the different types of formulas?

Answer 1

General formula - algebraic formula that can be applied to chemicals in the family
Molecular formula - The actual number of atoms in a molecule or element so ethane is C2H6
Empirical formula - Simplest whole number ratio of atoms in a compound
Structural formula - the arrangement of atoms in a molecule without showing bonds CH3CH3CH3…
Skeletal formula - shows bonds of the carbon skeleton only
Displayed formula - shows all the bonds in a molecule

Question 2

What is a homologous series?

Answer 2

Group of compounds that have the same functional group and general formula
Successive members increase by CH2 every time

Question 3

Nomenclature

Answer 3

1. Find the length of the stem by counting th largest continuous chain
2. The functional group on the molecule normally tells you the ending of the name (suffix)
3. Number the carbon chain so that the functional group sits on the lowest possible number carbon
4. Make a note of the carbon number the functional group is attached to, place this number before the suffix.
5. Any side chains and less important functional groups are written as prefixes in alphabetical order
6. If there is more than 1 identical functional group or side chain, but di or tri or tetra

Question 4

Classifying reactions

Answer 4

Polymerization - joining smaller monomers together to form a very long chain
Addition - joining molecules together to form a larger one
Hydrolysis - splitting a molecule into 2 using OH- or H+ from a H2O molecule
Substitution - Replacing one atom with another molecule
Elimination - Some atoms break away from a larger molecule
Reduction - when a species gains electrons in a reaction
Oxidation - when a species loses electrons in a reaction

Question 5

What do mechanisms look like?

Answer 5

Shows the movement of electrons during a chemical reaction
Use curly arrows to show the movement of a pair of electrons
Always starts from an electron-dense area and ends where you are moving the electrons to or the formation of a new bond.

Question 6

What are the types of mechanisms?

Answer 6

Free radical substitution - used when halogens react with alkanes to form halogenoalkanes
Electrophilic addition - used in adding halogens and hydrogen halides to alkenes to make halageno alkanes
Nucleophilic substitution - Used in primary halogenoalkanes and aqueous KOH to make alcohols. Amines are made by reacting with ammonia

Question 7

What are the different types of structural isomerism?

Answer 7

Structural isomers have the same molecular formula but different structural formula

Chain isomers - Same molecular formula but different arrangement of the carbon skeleton
Positional isomer - Same molecular formula but different position of the fiunctional group on the carbon skeleton
Functional group isomer - Same molecular formula but different functional group

Question 8

What are alkanes?

Answer 8

Saturated hydrocarbons - CnH2n+2
Saturated which means carbon is bonded 4 times with the maximum number of hydrogens possible
Cycloalkanes are slightly different they have a general formula of CnH2n with 2 hydrogens per carbon

Question 9

What are the 2 types of bond fission?

Answer 9

It is the breaking of a covalent bond and the electron pair can be distributed in 2 ways

Heterolytic fission is when electrons are distributed unequally to 2 different ions. The double-headed arrow shows the movement of a pair of electrons

Homolytic fission is when electrons in the bond are being shared equally to form 2 charged radicals

Question 10

What are the steps in a free radical substitution (making halogenoalkanes) Methane and chlorine.

Answer 10

1. Initiation -
   Sunlight breaks the cl–cl bond in a process called photodissociation. The bond breaks equally producing 2 highly reactive radicals
   Cl – Cl —–> Cl. + Cl.
2. Propagation -
   The Cl radical reacts with a methane molecule to make a methyl radical
   The methyl radical reacts with a Cl2 molecule forming chloromethane and a Cl. radical
   This Cl. radical can react with more methane
   Cl. + Ch4 —-> Hcl + CH3.
   Ch3. + cl2 —-> Ch3Cl + Cl.

Termination -
2 radicals react to make a stable non radical molecule
Ch3. + Cl. —–> Ch3Cl

Question 11

What are the different products of free radical reactions?

Answer 11

Producing just chloromethane is difficult as further reactions occur producing a mixture of products
If there is a lot of chlorine in the reaction mixture, we get di tri and tetra haloalkane
Cl. + Ch3Cl —-> .Ch2Cl + Hcl
.Ch2Cl + Cl2 —-> .Ch2Cl2 + Cl.
Dichloromethane is formed. This can also react further and make trichloromethane. Tri can react further to make tetra.

Have to be separated depending on what we want. You can add excess methane to reduce the amount of multiple substitution

Question 12

What is fractional distillation?

Answer 12

Crude oil is vaporised using a furnace to about 350 degrees
The vaporised oil enters the column and ruses through the trays. The longest hydrocarbons don’t vaporise and stay at the bottom

The column has a temp gradient, it is cooler at the top. As the vapour rises parts of the mixture condenses at different temperatures as there are different chain lengths and hence different BP’s

Heavier fractions can be cracked to higher demand for lighter fractions

Question 13

What is cracking? Thermal and catalytic

Answer 13

Heavier fractions can be cracked to higher demand for lighter fractions

Thermal cracking:
100 degrees and 70atm
Alkenes are used to make polymers such as plastic
Products of thermal cracking are mainly alkenes

Catalytic cracking:
450 degrees and slight pressure
Zeolite catalyst used to lower temp
Products are mainly aromatic hydrocarbons useful in fuels for vehicles

Question 14

How can you reform alkanes?

Answer 14

They can be reformed into cycloalkanes and aromatic molecules
The problem is that straight-chain alkanes create knocking in a car engine. Knocking is when straight-chain alkanes explode upon compression.
Adding branched and cyclic alkanes reduces knocking and increases engine efficiency
Cyclic and branched alkanes can be made using straight-chain alkanes and a platinum catalyst in a process called reforming
Hexane is reformed to cyclohexane and then benzene

Question 15

Complete combustion of alkanes

Answer 15

They burn in oxygen completely to form CO2 and H2O.
They’re very good fuels as most burn readily to produce large amounts of energy
The larger the alkane the more energy they produce

Question 16

Incomplete combustion of alkanes

Answer 16

When alkanes burn with limited oxygen they produce CO and C.
Carbon monoxide is poisonous as it bonds with haemoglobin in the blood and prevents oxygen bonding. Carbon monoxide can be removed using a catalytic converter.
Soot can cause breathing problems, make buildings dirty and clog up engines

Question 17

What is acid rain?

Answer 17

Burning fossil fuels can release SO2 and oxides of nitrogen which contribute to acid rain. It can cause damage to plants, kill fish, and cause erosion of buildings.
SO2 is acidic which can react with water in the atmosphere to form sulfuric acid which falls as acid rain
Oxides of nitrogen are formed when nitrogen and oxygen from the air are heated in the engine and react

Question 18

What are catalytic converters?

Answer 18

Found mainly in vehicles and help to reduce harmful pollutants entering the atmosphere
Help reduce the amount of unburnt hydrocarbons and oxides of nitrogen from going into the atmosphere
They convert harmful gases into less harmful products like water vapor, nitrogen and oxygen

Question 19

What are the advantages and disadvantages of Biofuels

Answer 19

Advantages:
Renewable
Produce CO2 when burnt but are classed as carbon neutral as the CO2 is absorbed by the plants when it’s growing

Disadvantages:
Expensive to convert existing petrol engines to take fuels with a higher concentration of ethanol
Land that can be used to grow food is being used for oil - food shortages

Question 20

What are sigma and pi bonds?

Answer 20

The double bond contains a sigma bond and a pi bond

In a sigma bond, that is when 2s orbitals overlap. Strong electrostatic attraction between the nuclei and shared pair of electrons due to the high electron density between nuclei. This makes the sigma bond have a high bond enthalpy

In a pi bond, it is the parallel overlap of 2p orbitals. They are dumbbell-shaped, and when they merge they form 2 oblong shapes. One on the top and one on the bottom. Pi bonds are weaker as the electron density is spread out above and below the nuclei. The electrostatic attraction is weaker so Pi bonds have a lower bond enthalpy

Question 20

Reactivity of alkenes

Answer 20

The double bonds in alkenes have a high electron density, It’s the high electron density area that makes them reactive. 4 electrons are being shared in the double bond.
Alkanes only have sigma bonds which makes them non-polar and so not very reactive compared to alkenes.
Pi bonds stick out a little and the whole double bond has a high electron density which means they are open to attack from electrophiles. They also have low bond enthalpy which is another reason why alkenes are reactive

Question 20

What are alkenes?

Answer 20

Unsaturated with CnH2n which means a least 1 double covalent bond
Double bonds have a very high electron density which makes them reactive
Cycloalkenes have 2 fewer hydrogens than their straight-chain version of the same molecule

Question 21

What is E/Z stereoisomerism?

Answer 21

Same structural formula but a different arrangement of atoms in space
Atoms can’t rotate around the C=C bond and is rigid. E/Z isomerism happens when you have 2 different groups of atoms on the same carbon
E = opposite side
Z = same side

Question 22

What are the CIP rules?

Answer 22

We use this when we have 4 different groups around the double bond
1) Label carbons with the double bond as 1 and 2
2) Calculate the atomic number of the first element directly bonded to the c=c bond. The atom with the highest atomic number is given a higher priority

Question 22

Electrophilic addition

Answer 22

The double bond in an alkene has a high electron density and is attacked by electrophiles.
They are electron pair acceptors. They are deficient in electrons and are attracted to the double bond
Examples are H+ or No2+ or HBr or H2SO4

Question 23

Test for alkenes (addition of bromine)

Answer 23

Adding bromine water to an alkene can cause a colour change from brown/orange to colourless
Bromine is the electrophile and adds to the alkaline forming a dibromoalkane

Br-Br is polarised as the electrons in the double bond repels the electrons in Br2. An electron pair in the double bond is attracted to delta + bromine and forms a bond. This breaks the br-br bond. A carbocation is formed and Br- is attracted to C+.

Question 24

Hydration of alkenes

Answer 24

Alcohols are produced by the hydration of alkenes
We use steam and an acid catalyst to create an alcohol
We can make ethanol by reacting steam and ethene with a phosphoric acid catalyst at a temp of 300 degrees and pressure of 60 atm is needed

Question 25

Forming alcohols from alkenes (without steam)

Answer 25

Adding acidified potassium manganate solution to an alkene will lead to a diol
Ethene —-> Ethane-1,2- diol

Question 26

Alkenes to halogenoalkanes?

Answer 26

Add hydrogen halides to the alkene
Reacting hydrogen halides with unsymmetrical alkenes produces 2 different products.
The amount of the 2 products is determined by the stability of the carbocation intermediate
The more alkyl groups bonded to the carbocation the more stable the intermediate is

Question 27

What are addition polymers?

Answer 27

Alkenes are monomers that join to make addition polymers
Most polymers aren’t biodegradable