Topic 10: Core Organic Chemistry

Question 1

Define saturated

Answer 1

Compounds that contain only single bonds. They have the maximum amount of atoms bonded to each carbon.
Eg. Alkanes

Question 2

Define unsaturated

Answer 2

Compounds which contain double or triple bonds.
Eg. Alkenes, arenes
(Note: compounds that are unsaturated are highly reactive)

Question 3

Define aliphatics

Answer 3

Compounds which do not contain a benzene ring (i.e. straight-chained). May be saturated or unsaturated.
Eg. Alkanes, alkenes

Question 4

Define arenes

Answer 4

Compounds which contain a benzene ring. All unsaturated.
Eg. Benzene, phenol (Phenyl functional group)

Question 5

Define electrophile

Answer 5

An electron-deficient species that is attracted to parts of molecules that are electron RICH. Attracted to the negative charge. Electrophiles are positive ions or have a partial positive charge.

Question 6

Define nucleophile

Answer 6

An electron-rich species that is attracted to parts of molecules that are electron DEFICIENT. Attracted to the positive charge. Nucleophiles have a lone pair of electrons or have a negative charge.

Question 7

Define homolytic fission

Answer 7

When a covalent bond breaks by splitting the shared pair of electrons between the two products. Both atoms get the same number of electrons.

X : X –> X˙ + X˙

Question 8

Define heterolytic fission

Answer 8

When a covalent bond breaks with both of the shared electrons going to one of the products.

X : X –> X :⁻ + X⁺

Question 9

What does a double-barbed arrow represent?

Answer 9

↷It represents the motion of an electron pair.
(Drawn from the site of electron availability to the site of electron deficiency)

Question 10

What does a single-barbed arrow represent?

Answer 10

It represents the motion of a single electron.
(Drawn from the site of electron availability to the site of electron deficiency)

Question 11

Define homologous series

Answer 11

A group of compounds with similar structural features that can be represented with a general formula.

Question 12

What do the physical properties of organic compounds depend on?

Answer 12

The physical properties, especially melting and boiling point, depend on the strength of the intermolecular forces. This is relative to the carbon chain length. The longer the carbon chain, the stronger the intermolecular forces therefore the higher the melting and boiling point.

The functional group also tells us about physical properties (eg. polarity/hydrogen bonding).

Question 13

Define the empirical formula

Answer 13

The simplest whole number ratio of the atoms in a compound. eg. C₂H₆ is CH₃

Question 14

Define the molecular formula

Answer 14

The actual number of atoms of the different elements in one molecule of the compound.

Question 15

Give the number of bonds needed for:
Hydrogen
Oxygen
Nitrogen
Carbon
Halogens

Answer 15

Hydrogen - 1
Oxygen - 2
Nitrogen - 3
Carbon - 4
Halogens - 1

Question 16

Give the general formula for Alkanes

Answer 16

CₙH₂ₙ+₂

Question 17

Give information about the use of alkanes as fuels

Answer 17

Alkanes are used as fuels because they release significant amounts of energy when they burn (the reaction is highly exothermic).

Complete combustion:
Alkane + excess oxygen —> carbon dioxide + water

Incomplete combustion:
Alkane + limited oxygen —> carbon monoxide + water
Alkane + very limited oxygen —> carbon + water

*The more carbon atoms present the more energy produced (but more oxygen is needed for complete combustion)

Question 18

What is the environment impact of the use of alkanes as fuels?

Answer 18

Complete combustion:
Carbon dioxide and water are both greenhouse gases and contribute to global warming.

Incomplete combustion:
Carbon monoxide is a toxic gas and can be fatal if produced in a confined space.

Carbon particulates are detrimental to the respiratory system in humans. They also act as catalysts to produce smog in polluted air.

Question 19

What is the main type of reaction that alkanes undergo?

Answer 19

Free radical substitution (halogenation). This is where a halogen takes the place of a hydrogen atom in the alkane.

There are three steps:
1) Intiation (photochemical homolytic fission)
Occurs in the presence of UV light (this is needed to overcome the strong bonds within the alkane). Two free radicals are produced (i.e. species with an unpaired electron).

2) Propagation (a chain reaction)
Free radicals are used and produced. Starts with 1radical and 1normal molecule and ends with 1radical and 1normal molecule.

3) Termination
Free radicals react together and pair up their electrons.

Question 20

Cycloalkane reactivity

Answer 20

Smaller cycloalkanes (cyclopropane and cyclobutane) are more reactive because their bond angles are distorted so they are under considerable strain.

Question 21

Define structural isomers

Answer 21

Structural isomers have the same molecular formula as the compound but a different structural formula (i.e. a different arrangement of atoms).

Question 22

Give evidence to support the delocalization structure of Benzene (MORE TO COME)

Answer 22

1) Bond lengths
All carbon-carbon bond lengths are of equal length and equal strength. They are intermediate between a single and a double bond.

2) Type of reactivity
Benzene undergoes substitution reactions, rather than addition reactions. (Addition reactions would disrupt the cloud of delocalised electrons and the product would be less stable. Substitution reactions preserve the ring structure).

Question 23

Give the general formula for alkenes + information on bonds

Answer 23

CₙH₂ₙ
Alkenes have a double bond.

Question 24

Give the reactions of the alkenes

Answer 24

Alkenes are more reactive than alkanes because the double bond is a site of reactivity. They undergo addition reactions.

1) Hydrogenation
Alkene + H₂ —> Alkane

2) Reaction w a halogen (eg. Br₂, I₂, Cl₂, Fl₂)
Alkene + Cl₂ —> Dihaloalkane

3) Reaction w hydrogen halides
Alkene + HCl —> Haloalkane
* Note the hydrogen halide HF is the least reactive (reactivity of the hydrogen halides increases as you go down G17)

4) Hydration
Alkene + H₂0 —> Alcohol
* Conditions: steam + concentrated sulfuric acid as a catalyst.

Question 25

What is the test used to distinguish an alkane from an alkene?

Answer 25

The bromine test.
A solution of bromine will change from orange to colourless if the molecule is unsaturated (i.e. an alkene because it will react with the bromine).

Question 26

What is the economic importance of the reactions of alkenes?

Answer 26

• The hydrogenation of vegetable oils produces margarine.
• The hydration of alkenes produces ethanol (used in alcoholic beverages).
• The addition polymerisation reaction of the alkenes produces plastics used in everyday life.

Question 27

Give the general formula for alkynes + information on bonds

Answer 27

CₙH₂ₙ₋₂
Alkynes have triple bonds.

Question 28

Give the general formula for aldehydes + information on bonds and its suffix

Answer 28

R-CHO
The functional group is carbonyl (C=O)
-anal

Question 29

Give the general formula for ketones + information on bonds and its suffix

Answer 29

R-CO-R’
The functional group is carbonyl (C=O)
-one

Question 30

Give the general formula for alcohols + information on bonds and its suffix

Answer 30

CₙH₂ₙ₊₁OH
The functional group is hydroxyl (OH)
-ol

Question 31

Give the reactions of the alcohols

Answer 31

• Complete and incomplete combustion
• Oxidation reactions

Question 32

Describe the primary alcohols oxidation reaction

Answer 32

• Oxidising agents involved are H⁺/KMnO₄ and H⁺/K₂Cr₂O₇

Primary alcohol —> Aldehyde —> Carboxylic acid
The aldehyde and carboxylic acid are obtained through different experimental set ups.

To isolate the aldehyde, the primary alcohol is placed over heat with the oxidising agent. Aldehydes have a lower boiling point so vaporise and rise. The vapor then passes horizontally through a condensing tube (a tube surrounded by cold water) where it condenses into a liquid and is collected.

To isolate the carboxylic acid, the primary alcohol is placed over heat and reflux with the oxidising agent. Aldehydes have a lower boiling point so vaporise and rises. It rises vertically into the condensing tube where it condenses and falls back into the flask, further oxidising to produce carboxylic acid.

Question 33

Describe the secondary alcohols oxidation reaction

Answer 33

• Oxidising agents involved are H⁺/KMnO₄ and H⁺/K₂Cr₂O₇

Secondary alcohol —> Ketone

Question 34

Describe the tertiary alcohols oxidation reaction

Answer 34

Tertiary alcohols do not oxidise.

Question 35

Describe the colour changes of the oxidising agents used in the oxidation of alcohols

Answer 35

H⁺/KMnO₄ (Acidified potassium permanganate)
H⁺/K₂Cr₂O₇ (Acidified potassium dichromate)

In acidic solution, potassium permanganate changes colour from purple to colourless.
In acidic solution, potassium dichromate changes colour from orange to green.

Question 36

Give the general formula for carboxylic acids + information on its bond and its suffix

Answer 36

CₙH₂ₙ₊₁COOH
The functional group is carboxyl (COOH) in which the C is double bonded to the O and single bonded to OH.
-anoic acid

Question 37

Give the general formula for esters + information on its bond and its suffix

Answer 37

RCOOR’
The functional group is carbonyl (C=O)
-anoate
Weird wonky structure. Name alkyl group first.

Question 38

Describe the esterification reactions

Answer 38

Alcohol + carboxylic acid -> ester + water
Remember “alcohol comes first”

Catalyst used: concentrated sulfuric acid
Condensation reaction (in equilibrium)

Question 39

Give the general formula for halogenoalkanes + information on its bonds and its suffix

Answer 39

R-X (where X represents a halogen)
Name in alphabetical order

Question 40

Describe the reactions of the halogenoalkanes

Answer 40

The halogen atom is more electronegative than the carbon so exerts a stronger pull on the shared electrons in the carbon-halogen bond. So the halogen becomes delta negative and the carbon delta positive. So the carbon is said to be electron deficient (an electrophile).

Reacts with alkalis to form alcohols.

Question 41

Describe the reactions of Benzene

Answer 41

Electrophilic substitution reactions (remember there are e’s in benzene!)
The delocalised electrons in benzene represent an area of electron density so is the site of reactivity. Substitution reactions occur more readily because the arene ring is conserved.