C9 Introduction to Organic Chemistry

Question 1

Explain why organic compounds are relatively stable.

Answer 1

This is due to the:

1) Lack of lone pairs of electrons in the skeletal chains
2) Inability of carbon to expand its valence shell to accommodate more than 8 electrons
3) Presence of strong C-C and C-H bonds.

Note: C-C is the strongest single bond between atoms of the same element

Question 2

Explain how to assign oxidation numbers to Carbon atoms in organic compounds.

Answer 2

Changes in the oxidation number of carbon in organic compounds indicate whether the organic compound has undergone an oxidation or reduction reaction.

1) In a compound, the more electronegative atom is given a negative oxidation number as bonding electrons are assigned to the more electronegative atom. For every bond, the more electronegative atom “gains” an electron and its oxidation state decreases by 1. The less electronegative atom “loses” an electron and the oxidation state increases by 1.
2) Sum of the oxidation numbers in a molecule = 0
3) Carbon is less electronegative than F, O, N, Cl, Br, I, S
4) If a bond is formed between two atoms of the same element, both atoms will have the same electronegativity and there is no “gain” or “loss” of electrons. (Useful for compounds where an element occurs many times and would have different oxidation states - usually average oxidation number would be found)

Question 3

Name the three types of hydrocarbons.

Answer 3

1) Aliphatic hydrocarbons
2) Alicyclic hydrocarbons
3) Aromatic hydrocarbons

Question 4

What are aliphatic hydrocarbons?

Answer 4

Aliphatic hydrocarbons are open chains of C atoms that contain single or multiple C-C bonds. They consist of straight-chain hydrocarbons, branched hydrocarbons, saturated and unsaturated hydrocarbons.

Question 5

What are alicyclic hydrocarbons?

Answer 5

Alicyclic hydrocarbons contain a closed ring of C atoms and single or multiple bonds, where the H atoms are typically not shown in the structure.

Question 6

What are aromatic hydrocarbons?

Answer 6

Aromatic hydrocarbons contain a benzene ring.

Question 7

Define a functional group.

Answer 7

A functional group is made up of an atom or group of atoms within the organic compound that is responsible for its characteristic chemical properties.

Question 8

Define a homologous series.

Answer 8

A homologous series is a family of compounds having the same functional group. Homologues refer to the members in the same homologous series.

Question 9

State the properties of homologues.

Answer 9

Homologues:

1) can be represented by a general formula
2) differ from the successive members by methylene -CH2- group
3) shows a gradual change in physical properties such as boiling points
4) have similar chemical properties
5) can be prepared by similar methods

Question 10

What is the simplest homologous series?

Answer 10

Alkanes which have no functional group.

Question 11

In what ways can organic compounds be represented?

Answer 11

Pg 8 of notes

1) Empirical formula
2) Molecular formula
3) Structural formula (includes: Displayed/Full structural, Condensed, Skeletal, Stereochemical)

Question 12

Recall the steps to drawing a skeletal formula.

Answer 12

Pg 9 to 11 of notes

Question 13

Recall the classes of compounds and their homologous series with the structure of the functional group and its general formula.

Answer 13

Pg 6 to 7 of notes

1) Hydrocarbons (Alkane, Alkene, Alkyne)
2) Halogen derivatives (Halogenoalkane, Halogenoarene)
3) Hydroxy compounds (Alcohol/hydroxy, Phenol/phenyl)
4) Carbonyl compounds (Aldehyde, Ketone)
5) Carboxylic acids and derivatives (Carboxylic acid, Ester, Acyl halide)
6) Nitrogen compounds (Amine, Amide, Amino acid, Nitrile)

Question 14

Recall how to name organic compounds.

Answer 14

Pg 12 to 16

Question 15

Define isomerism.

Answer 15

Isomerism refers to the existence of two or more compounds with the same molecular formula but different arrangement of atoms in their molecules. These compounds are known as isomers.

Question 16

Define the two types of isomerism.

Answer 16

Constitutional (structural) isomerism have the same molecular formula but different structural formula (arrangement of atoms).

Stereoisomerism refers to the existence of two or more compounds with the same molecular formula and structural formula but different SPACIAL arrangement of atoms.

Question 17

Define the three types of constitutional isomerism. State whether their chemical and physical properties differ from one isomer to another.

Answer 17

There are three main types of constitutional isomers:

1) Chain Isomers: Differ in the arrangement of carbon atoms in the chain. Carbon atoms may be arranged in straight chain or branched chain. Since chain isomers have the same functional groups, they have similar chemical properties and still undergo the same reactions. However, they have different physical properties. For instance, the difference in boiling points is due to the differences in surface area affecting the strength of the id-id interactions.
2) Positional Isomers: Have the same homologous series with the same functional group located at different positions in the same carbon skeleton. Since they have the same function groups, they have similar chemical properties but different physical properties.
3) Functional Group Isomers: Have different functional groups and thus different chemical and physical properties.

Question 18

State the two types of stereoisomerism.

Answer 18

1) Cis-Trans Isomerism

2) Enantiomerism

Question 19

State the 4 pairs of functional group isomers.

Answer 19

Alcohol and Ether
Aldehyde and Ketone
Carboxylic acid and Ester
Alkene and Cycloalkane

Question 20

State the two conditions that must be met for Cis-Trans Isomerism to exist in alkenes and cyclic compounds.

Answer 20

For alkenes, there must be restricted rotation about a bond by the presence of a double bond. Each C atom in the C=C bond must have two different groups attached to it.

For cyclic compounds, there must be restricted rotation about a bond by the presence of the ring structure. Two or more of the carbon atoms in the ring must have two different groups attached to them.

Question 21

Describe a cis-isomer and a trans-isomer.

Answer 21

A cis-isomer has two identical groups on the same side of the C=C bond.

A trans-isomer has two identical groups on the opposite sides of the C=C bond.

Question 22

Explain why there is restricted rotation in alkenes.

Answer 22

In alkenes, the C=C bond cannot undergo free rotation unless the pi bond in the C=C bond is broken.

Question 23

State the formula used to find the maximum number of cis-trans isomers a compound can have.

Answer 23

If the number of C=C bonds in a compound is n, the maximum number of cis-trans isomers for the compound is 2^n.

Question 24

Apart from similar chemical properties, compare the physical properties of a cis-isomer and a trans-isomer and account for these differences.

Answer 24

Compared to the trans-isomer, the cis-isomer has a higher boiling point because of it’s slightly higher polarity (very slight net dipole moment). Thus, it has a lower melting point because it fits into a crystalline lattice more poorly.

Question 25

Show how cyclic compounds can exhibit cis-teams isomerism.

Answer 25

Pg 21 of notes

Question 26

State the conditions for an organic compound to exhibit enantiomerism. State the steps to consider the existence if enantiomerism in a molecule.

Answer 26

A compound displays enantiomerism if it forms a non-superimposable mirror image. Such compounds are known as chiral compounds.

Pg 27 of notes

Question 27

State the properties of a chiral compound.

Answer 27

Chiral compounds:

1) do not have a plane of symmetry
2) usually contain one or more chiral centres
3) forms a non-superimposable mirror image

Question 28

Does a molecule with a plane of symmetry exhibit enantiomerism?

Answer 28

A molecule with a plane of symmetry forms a superimposable mirror image. It does not exhibit enantiomerism.

Question 29

What is a chiral carbon?

Answer 29

A chiral carbon is sp^3 hybridized and has four different groups attached to it. A molecule with no plane of symmetry and at least one chiral carbon forms non-superimposable mirror images and thus, exhibit enantiomerism.

Question 30

State the steps to draw the special arrangement of enantiomerism.

Answer 30

Identify the chiral carbon and place it at the centre of the structure.
Reflect the structure while drawing dotted lines in the middle of the two structures denoted as a mirror plane.

Question 31

Compare and contrast the properties of enantiomers.

Answer 31

Physical properties are identical except that they rotate the plane of plane-polarised light in equal but opposite directions.

Chemical properties are identical except in their interaction with another chiral molecule

Biological properties are different.

Question 31

Compare and contrast the properties of enantiomers.

Answer 31

Physical properties are identical except that they rotate the plane of plane-polarised light in equal but opposite directions.

Chemical properties are identical except in their interaction with another chiral molecule

Biological properties are different.

Question 32

Compare and contrast the properties of enantiomers.

Answer 32

Physical properties are identical except that they rotate the plane of plane-polarised light in equal but opposite directions.

Chemical properties are identical except in their interaction with another chiral molecule

Biological properties are different.

Question 33

Explain why non-chiral compounds do not rotate plane-polarized light.

Answer 33

In non-chiral compounds, the large number and distribution of molecules in a sample, each of the rotation by the individual molecules would he cancelled out and hence there is no net rotation or the plane-polarized light.

Question 34

State whether there is optical activity and rotation of the plane-polarised light in samples (i) containing one of the enantiomers and (ii) containing equal proportions of the two enantiomers/racemic mixture.

Answer 34

(i) In the sample containing one of the enantiomers, the plane-polarised light rotates the plane of plane-polarized light clockwise. The sample containing the other enantiomer will rotate the plane-polarised light by the same angle but anti-clockwise. The samples are both optically active.
(ii) In the racemic mixture containing equal proportion of both enantiomers, the rotating power of one enantiomer exactly cancels that of the other. It does not rotate the plane of plane-polarised light. The sample is not optically active.

Question 35

State the formula used to calculate the maximum number of stereoisomers a molecule can have given a certain number of chiral centres.

Answer 35

A molecule with n chiral centers has a maximum of 2^n stereoisomers.

Question 36

State the formula used to find the maximum number of stereoisomers a molecule can form given that it has x chiral centers and y double bonds that give rise to cis-trans isomerism.

Answer 36

2^(x+y)

Question 37

State the properties of a meso compound.

Answer 37

A meso compound:

• contains more than one chiral center but has a plane of symmetry
• has mirror images that are superimposable
• is optically inactive

Question 38

How is the degree of substitution of an atom in an organic compound determined?

Answer 38

The degree of substitution is determined by the number of alkyl or aryl groups bonded to it (i.e. Primary, Secondary, Tertiary, Quaternary)

Question 39

Define an electrophile.

Answer 39

An electrophile is defined as an electron pair acceptor (Lewis Acid). It is an electron-deficient species that are attracted to regions of negative charge or regions of electron-rich sites in a molecule. Such species may have a positive charge/a partial positive charge/incomplete octet. It may possess an empty low-lying/energetically accessible orbital to accept an electron pair.

Question 40

Define a nucleophile.

Answer 40

A nucleophile is an electron pair donor (Lewis Base). It is an electron-rich species that is attracted to regions of positive charge/regions of electron-deficient sites in a molecule/electron-deficient atom to form a covalent bond. It possesses at least one lone pair of electrons.

Question 41

Define a free radical.

Answer 41

A free radical is an atom or a group of atoms with an unpaired electron. It contains an unpaired electron and is electrically neutral.

Question 42

Describe the substitution reaction. Give examples of such reactions.

Answer 42

• During a substitution reaction, 1 atom or a group of atoms is replaced by another atom or group of atoms.
• 2 species react to give 2 products.
• A sigma bond breaks and another sigma bond forms at the same carbon atom.
• The degree of unsaturated remains unchanged
• The hybridisation of C remains unchanged.

Such reactions include Free Radical Substitution, Electrophilic substitution, Nucleophilic substitution.

Question 43

Describe the addition reaction. Give examples of such reactions.

Answer 43

• Addition reaction only occurs when there is unsaturation in the reactant molecules.
• During the addition reaction, 2 species react to form a single product.
• A pi bond is broken to form two new sigma bonds.
• Decrease in the degree of unsaturation.
• Hybridisation of carbon changes.

Such reactions include Electrophilic Addition and Nucleophilic Addition.

Question 44

Describe the elimination reaction.

Answer 44

• Elimination reaction involves the removal of atoms or groups of atoms from adjacent carbon atoms to form multiple bonds.
• 2 sigma bonds are broken and a pi bond is formed between adjacent atoms.
• The degree of unsaturation increases.

(Exact opposite of addition reactions)

Question 45

Describe the condensation reaction.

Answer 45

• During the condensation reaction, two molecules react together to form a bigger molecule with the elimination of small molecules such as H2O and HCl.
• The degree of unsaturation remains unchanged.

Question 46

Describe the hydrolysis reaction.

Answer 46

A reaction in which water is used to break a bond in a molecule.

Question 47

Describe the rearrangement reaction.

Answer 47

The rearrangement reaction involves the migration of an atom or groups of atoms from one site to another within the same molecule.

Question 48

Define the two ways in which a covalent bond can break.

Answer 48

Homolytic fission is the breaking of a covalent bond such that one electron goes to each of the atoms, forming free radicals. (It involves equally dividing the electrons between the two atoms in the bond such that one electron goes to each atom and the bond breaks.)

Heterolytic fission is the breaking of a covalent bond such that both electrons go to the same atom, typically forming positive and negative ions. (It involves unequally dividing the electrons between the two atoms such that two electrons go to the same atom and the bond breaks)

Question 49

Describe the two ways in which heterolytic fission can occur.

Answer 49

1) Both electrons can go to the group that is bonded to the carbon atom. This occurs when the carbon atom is less electronegative than X. The positively charged carbon ion formed is called carbocation.
2) Both electrons can go to the carbon atom. Often, this occurs when the carbon is more electronegative than X. The negatively charged carbon ion formed is called the carbanion.

Question 50

Explain why some ring structures cannot exhibit cis-trans isomerism.

Answer 50

Some ring structures cannot exhibit cis-trans isomerism because the trans-isomer is too unstable to exist due to ring strain. (Note: Ring structures are usually cis isomers.)

Question 51

How many pi electrons are there in a benzene ring?

Answer 51

3 times 2 equals 6 pi electrons

Question 52

Explain why 2,3 dibromobutane has 2 chiral centres but only 3 stereoisomers.

Answer 52

It has a plane of symmetry between C2 and C3. Thus, it has mirror images that are superimposable and optically inactive. For instance, two of its stereoisomers have plane of symmetry across C2 and C3, making them identical molecules and thus, meso compounds. As a result, there are only 3 stereoisomers.