Organic Chemistry and instrumental analysis

Question 1

There are two ways in which a bond can be broken. Name them.

Answer 1

Homolytic fission and heterolytic fission

Question 2

Describe homolytic fission.

Answer 2

Occurs when each atom retains 1 electron from the covalent bond and the bond breaks evenly, resulting in the formation of 2 neutral radicals.

Question 3

What type of bond will be broken homolytically?

Answer 3

Non polar covalent bond

Question 4

Describe heteolytic fission.

Answer 4

Occurs when 1 atom retains both electrons from the covalent bond and the bond breaks unevenly,

Question 5

What type of bond will be broken heterolytically?

Question 6

What does a single headed arrow and a double headed arrow indicate?

Answer 6

A single headed arrow indicates the movement of a single electron.
A double headed arrow indicates the movement of an electron pair.

Question 7

What does the tail of an arrow show?

Answer 7

The tail of the arrow shows the source of the electron(s).

Question 8

What does the head of the arrow indicate?

Answer 8

The head of the arrow indicates the destination of the electron(s).

Question 9

What do 2 single headed arrows starting at the middle of a covalent bond indicate?

Answer 9

Two single headed arrows starting at the middle of a covalent bond indicate homolytic bond fission is occurring.

Question 10

What does a double headed arrow starting at the middle of a covalent bond indicate?

Answer 10

A double headed arrow startling at the middle of a covalent bond indicates hetelytic bond fission is occurring.

Question 11

What does an arrow draw with the head pointing to the space between two atoms indicate?

Answer 11

An arrow drawn with the head pointing to the space between two atoms indicates that a covalent bond will be formed between those two atoms.

Question 12

What are nucleophiles?

Answer 12

Nucleophiles are negatively charged ions or neutral molecules that are electron rich, they are attracted towards atoms bearing a partial or full positive charge and are capable of donating an electron pai to form a new covalent bond.

Question 13

Give examples of nucleophiles.

Answer 13

Carbanions(C-), Cl-, OH-, H2O, Br-, CN-, NH3.

Question 14

What are electrophiles?

Answer 14

Electrophiles are positively charged ions or neutral molecules that are electron deficient, they are attracted towards atoms bearing a partial or full negative charge and are capable of accepting an electron pair to form a new covalent bond.

Question 15

Give examples of electrophiles.

Answer 15

Carbocations (C+), H+, NO2 (+).

Question 16

What is an addition reaction?

Answer 16

Molecule added across a C=C or C to C triple bond.
C2H4 +Br2 forms C2H4Br2

Question 17

What is an elimination reaction?

Answer 17

Removal of a molecule leaving behind a C=C double bond.
C2H5OH forms C2H4 + H2O
Remember: a molecule is removed not an atom.

Question 18

What is a condensation reaction?

Answer 18

Two molecules join together and a small molecule is removed.
CH3OH + CH3COOH forms CH3OCOCH3 + H2O
(Methanol + Ethanoic acid)

Question 19

What is a hydrolysis reaction?

Answer 19

Molecule splits into two molecules with the addition of a small molecule being added.
CH3OCOCH3 + H2O forms CH3OH + CH3COOH.
The small molecule added is water (H2O)

Question 20

What is a substitution reaction?

Question 21

What is an oxidation reaction?

Question 22

What is haloalkanes?

Answer 22

Haloalkanes (alkyl halides) are substituted alkanes in which one or more of the hydrogen atoms is replaced with a halogen atom.

Question 23

What is a primary monohaloalkane?

Answer 23

Carbon bonded to the halogen group is bonded to 1 other carbon. RCH2-X

Question 24

What is a secondary haloalkane?

Answer 24

Carbon bonded to the halogen is bonded to 2 other carbons. (R)2CH-X

Question 25

What is a tertiary haloalkane?

Answer 25

Carbon bonded to the halogen is bonded to 3 other carbon. (R)3C-X

Question 26

What type of reactions do monohaloalkanes take part in?

Answer 26

Elimination reactions with a strong base such as potassium or sodium hydroxide in ethanol to form alkenes. Nucleophilic substitution reactions with: - aqueous alkalis to form alcohols - alcoholic alkoxides to form ethers - ethanolic cyanide to form nitriles, which can be hydrolysed to form carboxylic acid.

Question 27

Which type of monohaloalkane is used in the SN1 mechanism?

Answer 27

Tertiary haloalkanes

Question 28

Why are tertiary haloalkanes used in SN1?

Answer 28

Tertiary haloalkane used because the alkyl groups are said to have an inductive effect which stabilises the C+ by donating electrons.

Question 29

Which haloalkanes are used for SN2 mechanism?

Answer 29

Primary or secondary haloalkanes.

Question 30

Why are tertiary haloalkane not used in SN2 mechanism?

Answer 30

Because of steric hinderance. If a tertiary haloalkane is used than a nucleophilic attack towards the carbon atom from the side opposite of the halogen is not possible, because of the 3 bulky alkyl groups will limit access to the C+ atom.

Question 31

What is formed when a primary alcohol is oxidised?

Answer 31

Aldehydes are formed when primary alcohols are oxidised and aldehydes can be furthered oxidised to carboxylic acids using: - Acidified potassium dichromate solution ( turns from orange to green) - Fehling’s solution (turns from blue to brick red) - Tollen’s reagent (turns from colourless to silver mirror effect)

Question 32

What reagents are used to oxidise primary alcohols into aldehydes?

Answer 32

Oxidising agents are used to oxidise primary alcohols, such as acidified potassium dichromate solution (turns from orange to green) or copper (II) oxide (from black to red)

Question 33

How are ketones formed?

Answer 33

Ketones are formed when secondary alcohols are oxidised.

Question 34

How are esters formed?

Answer 34

When alcohols reacts with carboxylic acids. This is a condensation reaction in which sulfuric acid is used as a catalyst to provide H+ ions.

Question 35

How can alcohols be prepared?

Answer 35

- haloalkanes by substitution, using aqueous alkali, such as sodium or potassium hydroxide in ethanol (either SN1 or SN2) - Reduction of aldehydes and ketones using lithium aluminium hydride (LiAIH4) dissolves in ether. - Hydration of alkenes using phosphoric acid as a catalyst

Question 36

What do alcohols react with?

Answer 36

-Alcohols react with reactive metals such as sodium and potassium to form alkoxides. - Alcohols can react with acid chlorides to form esters, as they react with carboxylic acids. This is a 2 way process: Stage 1- the acid is converted into an acid chloride by reaction with either thionyl chloride (SOCl2), phosphorus (V) chloride (PCl5) or phosphorus (lll) chloride (PCl3) Stage 2-the acid chloride formed reacts with an alcohol to form esters. - Dehydration to form alkenes using aluminium oxide, concentrated sulphuric acid or concentrated phosphoric acid. - formation of esters by reaction with carboxylic acids using concentrated sulphuric acid as a catalysts. - oxidation of primary alcohols to form aldehydes and then carboxylic acids and secondary alcohols to from ketones, using acidified permanganate, acidified dichromate or hot copper (ll) oxide.

Question 37

What type of bonding is present in alcohols?

Answer 37

Hydrogen bonding due the hydroxyl group that makes the alcohol polar. This gives alcohol high bp and mp.

Question 38

What is the functional group of ethers?

Answer 38

Functional group (alkoxy group) is -OR, and have a general structure R’-O-R”. R’ and R” are alkyl group.

Question 39

How are ethers formed?

Answer 39

Ethers are formed in a nucleophilic substitution reaction by reacting a monohaloalkane with an alkoxide.

Question 40

Why do ethers have low boiling points?

Answer 40

Due to the lack of hydrogen bonding between ether molecules, they have lower boiling points than the corresponding isomeric alcohols.

Question 41

Are ethers soluble in water?

Answer 41

Larger ethers are insoluble in water due to their increased molecular size, however methoxymethane and methoxyethane are soluble in water.

Question 42

Why ethers are used as solvents?

Answer 42

Because they are relatively inert chemically and will dissolve many organic compounds.