Module 4 (Core Organic Chemistry)

Question 1

What is a hydrocarbon?

Answer 1

Hydrocarbon is a compound consisting of hydrogen and carbon only

Question 2

What is the difference between saturated and unsaturated?

Answer 2

Saturated: Contains single carbon-carbon bonds only
Unsaturated: Contains a C=C double bond

Question 3

What is the molecular formula?

Answer 3

Molecular formula: The formula which shows the actual number of each type of atom

Question 4

What is the empirical formula?

Answer 4

Empirical formula: shows the simplest whole number ratio of atoms of each element in the compound

Question 5

What is the general formula?

Answer 5

General formula: algebraic formula for a homologous series e.g. CnH2n

Question 6

What is the structural formula?

Answer 6

The structural formula shows the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH3CH2CH2CH3 or CH3(CH2)2CH3

Question 7

What is the displayed formula?

Answer 7

Displayed formula: show all the covalent bonds present in a molecule

Question 8

What is the skeletal formula?

Answer 8

The skeletal formula shows the simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional Groups.

Question 9

What are Aliphatic, Alicyclic and Aromatic?

Answer 9

Aliphatic: a compound containing carbon and hydrogen joined together in straight chains, branched chains or
non-aromatic rings
Alicyclic: an aliphatic compound arranged in non-aromatic rings with or without side chains
Aromatic: a compound containing a benzene ring
Saturated: single carbon-carbon bonds only
Unsaturated: The presence of multiple carbon-carbon bonds, including C=C, C≡C and aromatic rings

Question 10

What is a homologous series?

Answer 10

Homologous series are families of organic compounds with the same functional group and the same general formula.
*They show a gradual change in physical properties (e.g. boiling point).
* Each member differs by CH2
from the last.
* same chemical properties.

Question 11

What is a functional group?

Answer 11

A functional group is an atom or group of atoms which when present in different molecules causes them to have similar chemical properties

Question 12

Rules for nomenclature with functional groups:

Answer 12

When compounds contain more than one functional group, the order of precedence determines which groups are named with prefixes or suffix forms. The highest precedence group takes the suffix (and the lowest number on the carbon chain), with all others taking the prefix form. However, double and triple C-C bonds only take suffix form.
Order of priority highest first:
Carboxylic acids >aldehydes>ketones>alcohols>alkenes>halogenoalkanes

Question 13

General rules for naming carbon chains:

Answer 13

Count the longest carbon chain and name it appropriately
Find any branched chains and count how many carbons they contain
 Add the appropriate prefix for each branch chain

Question 14

What is a structural isomer?

Answer 14

Structural isomers: same molecular formula different structures (or structural formulae)

Structural isomerism can arise from
*Chain isomerism
*Position isomerism
*Functional group isomerism

Chain isomers: Compounds with the same molecular formula but different structures of the carbon skeleton

Functional group isomers: Compounds with the same molecular formula but with atoms arranged to give different functional groups

Position isomers: Compounds with the same molecular formula but different structures
due to different positions of the same functional group on the same carbon skeleton

Question 15

What are Alkanes?

Answer 15

Alkanes and cycloalkanes are saturated hydrocarbons
General formula CnH2n

Question 16

Boiling Point of Alkanes:

Answer 16

The increasing boiling points of the alkane homologous series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the induced dipole-dipole interactions (London forces) between molecules.
The shape of the molecule can also affect the size of the induced dipole-dipole interactions (London forces). Long chain alkanes have a larger surface area of contact between molecules for London force to form than compared to spherical shaped branched alkanes and so have stronger induced dipole-dipole interactions and higher boiling points.

Question 17

Reactivity of Alkanes:

Answer 17

The low reactivity of alkanes with many reagents can be explained by the high bond enthalpies of the C-C
and C-H bonds and the very low polarity of the σ-bonds present.

Question 18

Hydrocarbons as fuels:

Answer 18

Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly
exothermic, explaining their use as fuels.

Complete Combustion
Incomplete combustion produces less energy per mole than complete combustion
The products of complete combustion are CO2 and H2O.
In excess oxygen, alkanes will burn with complete combustion
C8H18(g) + 12.5 O2(g) -> 8CO2(g) + 9 H2O(l)

Incomplete Combustion
If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame)
CH4(g) + 3/2 O2(g) -> CO(g) + 2 H2O(l)
CH4(g) + O2(g) -> C(s) + 2 H2O(l)

Carbon (soot) can cause global dimming- a reflection of the sun’s light

Carbon monoxide is a highly toxic but odourless gas. It can cause death if it builds up in an enclosed space due to faulty heating appliances.

CO is toxic to humans as CO can form a strong bond with haemoglobin in red blood cells. This is a stronger bond than that made with oxygen and so it prevents the oxygen from attaching to the haemoglobin.

Question 19

cracking:

Answer 19

Cracking: conversion of large hydrocarbons to smaller molecules of by breakage of C-C bonds
High Mr alkanes -> smaller Mr alkanes+ alkenes + (hydrogen)
This is a chemical process involving the splitting of strong covalent bonds so requires high temperatures.
Economic reasons for catalytic cracking:
* The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
* To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are
cracked.
* The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.)
Conditions:
Slight pressure
High Temperature (450°C)
Zeolite Catalyst

Catalytic Cracking Turns straight-chain alkanes into branched and cyclic alkanes and Aromatic hydrocarbons
Used for making motor fuels
Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number

Question 20

Substitution reactions of alkanes

Answer 20

Reaction of alkanes with bromine/chlorine in UV light
In the presence of UV light alkanes react with chlorine to form a mixture of products with the halogens substituting hydrogen atoms.
In general, alkanes do not react with many reagents. This is because the C-C bond and the C-H bond are relatively strong

Overall Reaction
CH4 + Cl2 -> CH3Cl + HCl
methane dichloromethane

This is the overall reaction, but a more complex mixture of products is formed

To understand this reaction fully we must look in detail at how it proceeds step by step. This is called its mechanism
The mechanism for this reaction is called a free radical substitution

It proceeds via a series of steps:
Step one: initiation
Step two: propagation
Step three: termination

Step one Initiation
Cl2 -> 2Cl. Essential condition: UV light
The UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to the others as it is the weakest.
The bond has broken in a process called homolytic fission.
each atom gets one electron from the covalent bond
When a bond breaks by homolytic fission it forms Free Radicals.
Free Radicals do not have a charge and are represented by a .
A Free Radical is a reactive species which possesses an unpaired electron

Step two Propagation
CH4 + Cl. -> HCl + .CH3
.CH3 + Cl2 -> CH3Cl + Cl.
The chlorine-free radicals are very reactive and remove an H from the methane leaving a methyl-free radical
The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical
All propagation steps have a free radical in the reactants and the products. As the Cl free radical is regenerated, it can react with several more
alkane molecules in a chain reaction

Step three Termination
.CH3 + Cl . -> CH3Cl .CH3 +
.CH3 -> CH3CH3
The collision of two free radicals does not generate further free radicals: the chain is terminated.

Question 21

What are Alkenes?

Answer 21

Alkenes are unsaturated hydrocarbons with the general formula: CnH2n

Question 22

Stereoisomerism in alkenes:

Answer 22

Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.
Alkenes can exhibit a type of isomerism called E-Z stereoisomerism
E-Z isomers exist due to restricted rotation about the C=C bond
Single carbon-carbon covalent bonds can easily rotate

E-Z stereoisomers arise when:
(a) There is restricted rotation around the C=C double bond.
(b) There are two different groups/atoms attached to both ends of the double bond.

E-Z stereoisomers can have differing melting and boiling points. As they may be polar or non-polar so have different intermolecular forces

Question 23

The reaction of Alkenes with Hydrogen:

Answer 23

Change in functional group: alkene -> alkane
Reagent: hydrogen
Conditions: nickel catalyst
Type of reaction: Addition/Reduction

The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions

Question 24

Reaction of alkenes with bromine/chlorine:

Answer 24

Change in the functional group: alkene -> dihaloalkane
Reagent: Bromine
Conditions: Room temperature (not in UV light)
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, Brδ+
Type of Bond Fission: Heterolytic

As the Br2 molecule approaches the alkene, the pi-bond electrons repel the electron pair in
the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+0).

The INTERMEDIATE formed, which has a positive charge on a carbon atom is called a CARBOCATION

Question 25

Reaction of Hydrogen Bromide with Alkenes:

Answer 25

Change in the functional group: alkene->haloalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic Addition
Type of reagent: Electrophile, Hδ+

HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond.

This reaction can lead to two products when the alkene is unsymmetrical

‘Markownikoff’s Rule’
In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it

If the alkene is unsymmetrical, the addition of hydrogen bromide can lead to two isomeric products.

Minor Product = 10%
MajorProduct = 90%

WHY?
This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron-releasing and reduce the charge on the ion which stabilises it.

The order of stability for carbocations is
tertiary > secondary >primary

In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.

Question 26

The reaction of alkenes with steam to form alcohols:

Answer 26

Industrially alkenes are converted to alcohols in one step. They are reacted with steam in the presence of an acid catalyst.

CH2=CH2 (g) + H2O (g) -> CH3CH2OH (l)
This reaction can be called hydration: a reaction where water is added to a molecule
Reagent: steam
Essential Conditions
High temperature 300 to 600°C
High pressure 70 atm
Catalyst of concentrated H3PO4

The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out

Question 27

Addition Polymers:

Answer 27

Addition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to
the strong C-C and C-H bonds

The industrial importance of alkenes:
The formation of polymers from ethene-based monomers is a major use of alkenes.
The manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils using hydrogen and a nickel catalyst is another important industrial process.

Liquid vegetable oils are generally polyunsaturated alkenes. Hydrogenation by the reaction of hydrogen using a nickel catalyst converts the double bonds to saturated single bonds. This increases the melting point of the oil making it harder and more solid.

Dealing with waste polymers:
Waste polymers can be processed in several ways.

Separation and recycling
The waste is sorted into different types of polymer (ie PTFE, PVC, PET) and then each type can be recycled by melting and remoulding.

Combustion for energy production
Waste polymers can be incinerated and the heat released can be used to generate electricity.

Combustion of halogenated plastics (ie PVC) can lead to
the formation of toxic, acidic waste products such as HCl. Chemists can minimise the environmental damage of this by removing the HCl fumes formed from the combustion process.

Feedstock for Cracking
Waste polymers can be used as a feedstock for the cracking process allowing for new production of plastics and other chemicals.

Chemists have also been developing a range of biodegradable polymers, compostable polymers, soluble polymers and photodegradable polymers.

Polymers formed from isoprene (2-methyl-1,3- butadiene), maize and starch are
biodegradable

Question 28

Alcohols:

Answer 28

General formula alcohols CnH2n+1OH
The alcohols have relatively low volatility due to their ability to form a hydrogen bond between alcohol molecules.
The smaller alcohols (up to 3 carbons) are soluble in water because they can form hydrogen bonds with water. The longer the hydrocarbon chain the less soluble the alcohol.

Uses of alcohols
Ethanol is ‘alcohol’ in alcoholic drinks. Ethanol is commonly used as a solvent in the form of methylated spirits. Methanol is used as a petrol additive to improve combustion and is increasingly important as a feedstock in the production of organic chemicals;

Question 29

Complete combustion of Alcohols:

Answer 29

Complete Combustion
CH3CH2OH (l) + 3 O2(g) -> 2CO2(g) + 3 H2O(l)
The products of complete combustion are CO2 and H2O.
In excess oxygen, alcohols will burn with complete combustion.

Question 30

Oxidation reactions of the alcohols

Answer 30

Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise.
The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions.

Partial Oxidation:
Reaction: primary alcohol -> aldehyde
Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.
Conditions: (use a limited amount of dichromate) Warm gently and distil out the aldehyde as it forms:
Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion

Full Oxidation of Primary Alcohols:
Reaction: primary alcohol -> carboxylic acid
Reagent: potassium dichromate(VI) solution and dilute sulfuric acid
Conditions: use an excess of dichromate, and heat under reflux: (distil off the product after the reaction has finished)
Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion

Question 31

What is Reflux?

Answer 31

Reflux:
Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids.

Question 32

Oxidation of Secondary Alcohols:

Answer 32

Reaction: secondary alcohol -> ketone
Reagent: potassium dichromate(VI) solution and dilute sulfuric acid.
Conditions: heat under reflux
Dehydration reaction: removal of a water molecule from a molecule

Question 33

Substitution reactions of Alcohols to form Haloalkanes:

Answer 33

A mixture of halide ions with concentrated acid NaCl + H2SO4 can used for substituting a halogen for an alcohol
CH3CH2OH + HCl -> CH3CH2Cl + H2O
Various other halogenating compounds can be used to substitute the –OH group for a halogen
NaCl + H2SO4 -> NaHSO4 + HCl
Reaction: Alcohol -> Haloalkane
Reagents: Concentrated sulfuric and sodium halide