Module 4: Basic Concepts of Organic Chemistry

Question 1

Define the term ‘displayed formula’

Answer 1

Shows the relative positions of all of the atoms in a molecule and the bonds between them

Question 2

Define the term ‘structural formula’

Answer 2

Gives the minimum detail for the arrangement of atoms in a molecule

Question 3

Define the term ‘skeletal formula’

Answer 3

The simplest way of representing organic molecules. Lines are used to indicate alkyl chains, and every corner represents a carbon atom

Question 4

Define the term ‘isomer’

Answer 4

A compound with the same molecular formula but a different structural formula

Question 5

State three types of isomer

Answer 5

Chain isomer, position isomer, functional group isomer

Question 6

What is a chain isomer?

Answer 6

Molecules have the same molecular formula but a different arrangement of the carbon skeleton (similar chemical properties but different physical properties)

Question 7

What is a position isomer?

Answer 7

Molecules have the same carbon skeleton but the functional group is in a different position (similar chemical properties but different physical properties)

Question 8

What is a functional group isomer?

Answer 8

Molecules have the same molecular formula but a different functional group (different chemical and physical properties)

Question 9

Define the term ‘homologous series’

Answer 9

A series of compounds with the same functional group but each successive member differs by CH2 e.g. alkanes

Question 10

Define the term ‘functional group’

Answer 10

A group of atoms responsible for the characteristics of a compound

Question 11

What is the alkyl group general formula?

Answer 11

CnH2n+1

Question 12

Define the term ‘aliphatic’

Answer 12

A compound containing hydrogen and carbon joined together in straight, branched or non aromatic rings e.g. propane

Question 13

Define the term ‘alicyclic’

Answer 13

Aliphatic compounds arranged in non aromatic rings or without side chains e.g. cyclohexane

Question 14

Define the term ‘aromatic’

Answer 14

A compound containing a benzene ring e.g. chlorobenzene

Question 15

What is a saturated hydrocarbon?

Answer 15

A compound which contains only single bonds

Question 16

What is an unsaturated hydrocarbon?

Answer 16

A compound which contains double or triple bonds

Question 17

What type of bonds are present in alkanes?

Answer 17

Sigma bonds

Question 18

Explain how sigma bonds are formed in alkanes

Answer 18

-Carbon is in group 4 so each atom has 4 electrons in its outer shell
-4 covalent bonds are formed because the 4 electrons pair up with electrons on other atoms
-Each covalent bond has direct overlap of the electron cloud from each atom which forms a sigma bond

Question 19

Explain the differences in boiling point as carbon chain length increases

Answer 19

-As carbon chain length increases, boiling point also increases because longer chains have more surface contact
-As there are more electrons, more London forces are produced which makes the attraction stronger
-More energy is needed to break the intermolecular London forces so the boiling point increases

Question 20

Explain the differences in boiling point of branched and unbranched hydrocarbons

Answer 20

-When a hydrocarbon is branched, the surface contact is less so there are fewer London forces
-Less energy is needed to break the intermolecular forces so boiling point decreases
-When a hydrocarbon is unbranched, there is more surface contact so there are more London forces
-More energy is needed to break the intermolecular London forces so boiling point increases

Question 21

State four physical properties of hydrocarbons

Answer 21

-Low melting and boiling point
-Shorter chains will be gasses at RTP
-No permanent dipoles as the electronegativity of C and H is similar to
-Only London forces (dependent on surface area)

Question 22

Explain why alkanes are unreactive

Answer 22

-C-C and C-H bonds are strong (have high bond enthalpy)
-C-C bonds are non polar
-C-H bonds are virtually non-polar because of similarities in the electronegativity of carbon and hydrogen

Question 23

Why are alkanes commonly used as fuels?

Answer 23

-Alkanes react with a supply of oxygen, releasing energy
-Complete and incomplete combustion

Question 24

State the general equation for complete combustion

Answer 24

Fuel + oxygen —> carbon dioxide + water

Question 25

Define the term ‘complete combustion’

Answer 25

When there is a plentiful supply of oxygen, the combustion of alkanes will always lead to the production of CO2 and water

Question 26

State the general equation for incomplete combustion

Answer 26

Fuel + oxygen —> carbon + carbon monoxide + water

Question 27

Define the term ‘incomplete combustion’

Answer 27

When there is a limited supply of oxygen, the combustion of alkanes will lead to the production of carbon monoxide (CO), soot (C) and water (H2O)

Question 28

Define the term ‘structural isomer’

Answer 28

A structural isomer has the same molecular formula but a different arrangement of atoms

Question 29

Define the term ‘homolytic fission’

Answer 29

In the presence of UV light the covalent bond in a diatomic halogen molecule will break and one of the bonded electrons goes to each atom forming two radicals

Question 30

State the three steps in free radical substitution

Answer 30

Step 1- Initiation (homolytic fission)
Step 2- Propagation
Step 3- Termination

Question 31

Explain what happens in step 1 (initiation) of free radical substitution

Answer 31

Homolytic fission- In the presence of UV light the covalent bond in a diatomic halogen molecule will break and one of the bonded electrons goes to each atom forming two radicals
E.g. Cl2 —> Cl. + Cl.

Question 32

Explain what happens in step 2 (propagation) of free radical substitution

Answer 32

The free radical reacts with a molecule to produce a new molecule and a new free radical
P1: CH4 + Cl. —> CH3. + HCl
P2: CH3. + Cl2 —> CH3Cl + Cl.

Question 33

Explain what happens in step 3 (termination) of free radical substitution

Answer 33

Two free radicals react to form a new molecule
CH3. + CH3. —> C2H6
Cl. + Cl. —> Cl2
Cl. + CH3. —> CH3Cl

Question 34

Why is a radical a highly reactive species?

Answer 34

It has an unpaired electron

Question 35

Define the term ‘radical’

Answer 35

A highly reactive species with an unpaired electron

Question 36

What does a ‘curly arrow’ represent in a reaction mechanism?

Answer 36

The movement of an electron pair

Question 37

Explain why free radical substitution is not an efficient way of making a particular product

Answer 37

-Substitution can occur at different points in a carbon chain
-The reaction produces a mixture of products
-More than one substitution can occur