Organic Chemistry

Disclaimer: This will only cover the SL content of the course.

Fundamentals of organic chemistry

Organic chemistry is the field in chemistry which studies carbon compounds. 

Carbon is able to combine with many different molecules including hydrogen which make up the basis of organic life. They can combine to make polymer chains, made up of c-c bonds, through a process called catenation

Cyclic: The carbon chain links back to itself to form a ring shaped structure

Degrees of saturation

Saturated compounds  contain only single bonds whereas Unsaturated compounds contain double or triple bonds. Only unsaturated compounds can react.

Homologous series

A homologous series is a series of compounds all of the same family which each differ by on structural unit. This allows us to group together the main compounds which exist in chemistry, into groups which have similar physical properties. 

Different homologous series

Trends in boiling points

For each successive structural unit, there will be a progressive increase in surface area and molecular mass. These are the factors which affect boiling point, therefore increasing these leads to an increase in boiling point

Image result for trends in boiling points between homologous series

Plotting number of carbon atoms vs boiling points gives a curved trendline, which is known in maths as the harmonic series. The reason it takes this shape is because each increase in structural unit there is a proportionally smaller effect on boiling point. 

Effect of branches on boiling point

A greater number of branches decreases the surface area, leading to a lower boiling point. For example a cyclic shaped compound should have a lower boiling point than one of a single chain.

Distinction between Empirical, Molecular and structural formula

Empirical formula: This is the ratio of the different elements which make up a compound. It does not give the structure or actual number of elements.

Molecular formula: This is the number of atoms of each element which make up a compound. This does not give structure.

Structural formula: This shows the arrangement of atoms in the element. Depending on the type of formula it can be presented either graphically or as text.

types of Structural formula 

Type Definition Diagram
Full This shows the actual arrangement of atoms in the molecule. It uses lines to show the bonds Image result for structural formula ethanoic acid
Condensed This gives the arrangement of the atoms in a  compound with only a single line of text. It uses parentheses to show groups of identical elements.
Skeletal

This is a graphical representation of the arrangement of elements used in organic chemistry to simplify the diagrams.

  • Points on the chain are carbons.
  • Hydrogen atoms are omitted, as it is assumed they fill in the free electrons on the carbons.
  • Carbons can be shown but only if part of a functional group.
  • Double bond is an extra line
Image result for skeletal formula of ethanoic acid

Isomerism

Structural Isomers are compounds with exactly the same molecular formula, but with different arrangements of the elements in the compound.

You can have:

Image result for isomerism example

Functional groups

Classes: These are the different types of compounds in organic chemistry. This is synonymous with "homologous series"

Functional groups: This is the reactive part of the molecule. It is responsible for the characteristic reaction of the different homologous series

Homologous series Suffix/prefix Functional group Bonding Example
Alkane -ane Alkyl group \(C-C\) Ethane
Alkene -ene Alkenyl group \(C=C\) Methene
Alkyne -yne Alkynl group \(C\equiv C\) Propyne
Halogenoalkane -oro  -alkane Halogen \(C-X\) Fluoroalkane
Alcohol -ol Hydroxyl group \(C-OH\) Ethanol
Arene -yl benzene Phenyl group Image result for r-benzene methyl benzene
Ether

-oxy -ane

Ether group Image result for Ether ethoxyethane
Ester -yl -oate Ester group Image result for ester group methyl ethanoate
Ketone -ketone Ketone group sugar
Carboxylic acid -oic acid Carboxyl group Image result for carboxyl group ethanoic acid
Aldehyde -al Aldehyde group Image result for aldehyde group ethanal
Amine -amine Amine group Image result for primary amine propamine
Amide -amide Amide group Image result for amide hexamide
Nitrile -nitrile Nitre group \(C\equiv N\) Acetonitrile

Naming organic molecules (IUPAC)

Essentials for naming

Detailed naming rules

Note: For the examples we will refer to the complicated looking ether above. (Don't worry you're unlikely to get one this difficult in the exam)

# What to identify Naming convention Example
1 Identify the parent carbon chain, which is the one which is longest

The length gives the root of the name.(meth, eth, prop, but, pent, hex, hept)

If two are the same length pick the one with the most side branches. 

  • The longest chain is 7 carbons long, so the prefix is hept-

hept-

2 Identify degrees of saturation e.g single, double, triple bounds.  This determines the part after the root. (-an, -en ,-yn)

The parent chain has two double bond so the suffix is -en.

hepten

3 Identify any functional groups

This determines the suffixes/prefixes. Only use a prefix if you can't use a suffix. 

  • OH: -ol (suffix)
  • COOH: -oic acid (suffix)
  • CH:- e (suffix)
  • OC: oxy- (prefix)
  • NH2-amine (suffix)

There is an oxygen atom so there is an ether (OC) functional group. There is also an alkyl group, Thus we add oxy- and -e

The ether group is part of a main chain of two carbon length so we also prepend eth-

ethoxyheptene

4 identify carbon side chains to main chain Add the names of the carbon chains as prefixes. These have the root which tells the number of carbons and the suffix which tells if their are double, or triple bonds.  Suffixes: single= yl, double=enyl: triple = yenyl)

There are no side chains.

5 Check if prefixes are in the right order If there are multiple prefixes they must be ordered alphabetically. They are already in alphabetical order, in this case.
6 Check if there is multiple of the same functional group/side chain/degrees of saturation  If there are multiples add prefix -di, -tri, -tetra to show how many

There are two double bonds so we add di- before the -en.

ethoxyhepta-di-ene (we add "a" because the primary suffix is followed by a constant)

7 Number carbon chain and then use the numbers to locate the side chains/functional groups/degrees of saturation The numbering chosen should be the one which results in the lowest total of the locations.
  • The ether group is on the 4th carbon from right or left
  • The double bonds are on the 2nd and 5th carbon
  • So name becomes 4-ethoxyhepta-2,5-diene
8 Exception to rule 7: If a functional group is present and must be at the end of the chain, then numbering must begin at this functional group.    

Benzene

This is a compound with the molecular formula \(C_6 H_6 \) Benzene is cyclic and forms a hexagon structure, where each carbon has one hydrogen. Each carbon has one double bond to another carbon and also one single bond.This creates a resonance structure

Related image

In skeletal form, to show the resonance structure, we put a circle inside the hexagon chain. This shows that the double bonds location can be in different places, hence the bond order is 1.5

Primary, secondary and tertiary compounds

Primary, secondary and tertiary are ways of classifying organic compounds

Alcohols and Halogenoalkanes

Whether an alcohol/halogenoalkane is primary, secondary or tertiary depends on the number of carbons which are bonded to the carbon which is bonded to the hydroxyl group/halogen

Classification Requirement Diagram
Primary One carbon bonded to the carbon bonded to the hydroxyl/halogen
Secondary Two carbons bonded to the carbon bonded to the hydroxyl/halogen
Tertiary Three carbons bonded to the carbon bonded to the hydroxyl/halogen

 

Amines

Whether an amine is primary, secondary or tertiary depends on the number of alkyls which are bonded to the nitrogen.

Classification Requirement Diagram
Primary One alkyl bonded to the nitrogen
Secondary Two alkyls bonded to the nitrogen
Tertiary Three alkyls bonded to the nitrogen

Functional group reactions

Alkanes

These are saturated hydrocarbons containing only carbon-carbon and carbon-hydrogen bonds. 

Reactivity of alkanes

Alkanes have low reactivity for two reasons:

Combustion reactions of alkanes (oxidisation)

Complete combustion: This is when alkanes are burnt in excess oxygen, to produce \(CO_2\) and \(H_2 O\). The (unbalanced) chemical equation is:  \(Alkane_{(g)} + {O_2}_{(g)} \longrightarrow {CO_2}_{(g)} + {H_2 O}_{(l)}\)

incomplete combustion: This is when alkanes are burnt with limited oxygen, to produce \(CO_2\) , \(H_2 O\) and \(CO\) or \(C\) . The (unbalanced) chemical equation is either \(Alkane_{(g)} + {O_2}_{(g)} \longrightarrow CO_{(g)} + {H_2 O}_{(l)}\) or \(Alkane_{(g)} + {O_2}_{(g)} \longrightarrow C (s) +{ H_2 O}_{(l)}\)

Free radical substitution

Free radical: An atom with unpaired electron which free to bond. The free radical is represented by a dot

Homolytic fission:splits the diatomic to produce two free radicals, with the same (homo) charge

This is a process where a halogen becomes a free radical and then substitutes for a hydrogen in the alkane. This process is a chain reaction and has three stages: initiation, propagation and termination.

  1. Initiation: A Halogen diatomic undergoes homolytic fission, which means it splits the diatomic to produce two free radicals. This is caused by UV light.
  2. Propagation: The free radical is very reactive and so can strip a hydrogen of the alkane. Forming a \(C-H\) molecule. The alkane is now a free radical and can cause other chlorine molecules to undergo homolytic fission. This creates a chain reaction
  3. Termination: The reaction terminates after all the free radicals come together to form stable molecules. This can happen in three ways:
    • Chlorine radical and alkane radical come together to form a halogenoalkane
    • Two chlorine radicals join together to form a \(Cl_2\) molecule
    • Two alkane radicals join to form a longer alkane.

Image result for Free radical substitution

Alkenes

Reactivity of alkenes

Alkenes are more reactive than alkanes because they have unsaturated double bonds.  This means that the double bond can become a single bond leaving space for the addition of other molecules. This is called an addition reaction.

Addition reactions

This is a reaction where the double bond is converted to a single bond allowing for the addition of other molecules.

In the IB course there are three different types of addition reactions covered:

Image result for Addition reactions

Addition polymerisation

Addition polymerisation is when the alkene combines with other alkenes through an addition reaction to form long chains called polymers. Polymers are very important because they are used to create plastics such as PVC of PTFE.

Image result for polymerisation of alkenes

Monomer: The alkene which undergoes polymerisation.

Repeating unit: this a part of the polymer which, when repeatedly combined, would form the full polymer. (excluding the end units).

Polymer: A long chain of monomers. Represented by the repeating unit surrounded by a bracket with a n in the bottom-right corner.

Alcohols

Combustion of alcohols

Like alkanes alcohols can undergo combustion reactions. You need to know how to write the equations for complete combustion, which is easy if you know that the products are again \(CO_2\) and \(H_2 O\)

\(C_2 H_6 O  + 3O_2 \longrightarrow 2 CO_2 + 3H_2O\)

Oxidation reactions

Alcohols can be oxidized into other functional groups, depending on whether they are primary, secondary or tertiary. An oxidizing agent such as potassium dichromate (K2Cr2O7) is used as oxygen does not split easily.

Obtaining aldehydes vs acids

To obtain an aldehyde from oxidizing a primary alcohol instead of a carboxylic acid the aldehydes must be removed from the reaction as soon as they are formed to prevent them oxidizing further. This can be done by taking advantage of the aldehydes lower boiling point compared to the alcohol. Here the alcohol is added to a boiling oxidizing agent so that as soon as the aldehyde forms it boils and can't be oxidized any further. Then a liebig condenser (Long tube with cold water) is add so that when the aldehyde gas passes through it, it will condense and can be collected.

The carboxylic acid can be obtained naturally, but to speed it up a boiling oxidizing agent is used and then a reflux condenser is added so that the evaporated aldehyde condense and goes back to the oxidizing agent, until it is fully oxidized.

Esterification

Esterification is when an alcohol and a carboxylic acid combine to form an ester. This reaction is a subcategory of condensation reactions, which are reaction that involve two molecules joining to form a large molecule. These reactions have steam as a by-product which is why they are called condensation reactions.

To combine the two molecules the Hydrogen at the end of the carboxylic acid and the OH from the alcohol are removed by a strong catalysts, so that they can join.

Note: Esters are volatile and are nice smelling. For this reason they are often used in perfumes or artificial flavourings

Halogenoalkanes

Nucleophile: a species with a lone pair of electrons and is thus attracted to positive species

In Halogenoalkanes the electronegative halogen forces the connecting carbon to have a positive charge. This makes it possible for Nucleophiles to replace the halogen, in a process called  nucleophilic substitution. 

(you don't need to memorize this diagram but it helps to visually understand nucleophilic substitution)

Equation which might come up: 

Benzene

Electrophilic substitution

Electrophile: A positively charged species which is attracted negative species.

In benzene, the ring structure gives it areas of high electron density above and below (why this happens is HL knowledge). Electrophiles are attracted to these negatively charged areas and can replace the hydrogens in benzene. This is electrophilic substitution

Evidence for resonance in benzene

 

Editors

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