4.2 Core Organic Chem 14.1-17.2

Question 1

The alcohol homologous series

Answer 1

Alcohols contain the –OH functional group, known as the hydroxyl group. The hydroxyl group is responsible for both the physical and chemical properties of the alcohols

Question 2

Methanol

Answer 2

Is the simplest alcohol, it is used as a high-performance fuel because of its efficient combustion. Methanol is also an important chemical feedstock – the starting material in many industrial syntheses. It can be converted into polymers, paints, solvents, insulation, adhesive, and many other useful products.

Question 3

Ethanol

Answer 3

The second member of the alcohol homologous series, it is used primarily in alcoholic drinks and as a fuel, and also finds use as a solvent and a feedstock

Question 4

Naming alcohols

Answer 4

The suffix –ol is added to the stem name of the longest carbon chain. The position of the alcohol functional group in the chain is indicated using a number

Question 5

Physical properties of alcohols

Answer 5

They are less volatile than alkanes, have higher melting points, and greater water solubility than the corresponding alkanes. The differences become much smaller as the length of the carbon chain increases.

Question 6

How can the differences in properties of alcohols and alkanes be explained by considering the polarity of the bonds in both?

Answer 6

– The alkanes have nonpolar bonds because the electronegativity of the hydrogen and carbon are very similar
– the alkane molecules are therefore nonpolar
– the intermolecular forces between nonpolar molecules are very weak London forces
– Alcohols have a polar O– H bond because of the difference in electronegativity of the oxygen and hydrogen atoms
– alcohol molecules are therefore polar
– the intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O– H groups

Question 7

Volatility and boiling points of alcohols

Answer 7

In the liquid state, intermolecular hydrogen bonds hold alcohol molecules together. These bonds must be broken in order to change the liquid alcohol into a gas. This requires more energy than overcoming the weaker London forces in alkanes, so alcohols have a lower volatility and the alkanes with the same number of carbon atoms

Question 8

Alcohol solubility in water

Answer 8

A compound that can form hydrogen bonds with water is far more water-soluble than a compound that cannot. Alkanes are non polar molecules and cannot form hydrogen bonds with water. Alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules
As the hydrocarbon chain increases in size, the influence of the -OH group becomes relatively smaller, and the solubility of longer chain alcohols becomes more like that of hydrocarbons- solubility decreases

Question 9

Classifying alcohols

Answer 9

Alcohols can be classified as primary, secondary, or tertiary. This classification depends on the number of hydrogen atoms and alkyl groups attached to the carbon atom that contains the alcohol functional group

Question 10

Primary alcohols

Answer 10

Methanol and ethanol

The -OH group is attached to one carbon atom that is attached to two hydrogen atoms and one alky group

Question 11

Secondary alcohols

Answer 11

The -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups

Question 12

Tertiary alcohols

Answer 12

The -OH group is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups

Question 13

Combustion of alcohols

Answer 13

Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water.
The reaction is exothermic, releasing a large quantity of energy in the form of heat. As the number of carbon atoms in the alcohol chain increases the quantity of heat released per mole also increases

Question 14

Oxidation of alcohols

Answer 14

Primary and secondary alcohols can be oxidised by an oxidising agent. The usual oxidising mixture is a solution of potassium dichromate(VI), acidified with dilute sulfuric acid. If the alcohol is oxidised, the orange solution containing dichromate(VI) ions is reduced to a green solution containing chromium (III) ions

Question 15

Oxidation of primary alcohols

Answer 15

Primary alcohols can be oxidised to either aldehydes or carboxylic acids. The product of the oxidation depends on the reaction conditions used because aldehydes are themselves also oxidised to carboxylic acids

Question 16

Preparation of aldehydes

Answer 16

On gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed. To ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms. This prevents any further reaction with the oxidising agent. The dichromate (VI) ions change colour from orange to green

Question 17

Preparation of carboxylic acids

Answer 17

If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate (VI), a carboxylic acid is formed. Use of an excess of the acidified potassium dichromate (VI) ensures that all of the alcohol is oxidised. Heating under reflux ensures that any aldehyde formed initially in the reaction also undergoes Oxidation the carboxylic acid

Question 18

Oxidation of secondary alcohols

Answer 18

They are oxidised to ketones. It’s not possible to further oxidise ketones using acidified dichromate (VI) ions.
To ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture. The dichromate (VI) ions once again change colour from orange to green.

Question 19

Oxidation of tertiary alcohols

Answer 19

They don’t undergo oxidation reactions. The acidified dichromate (VI) remains orange when adds to a tertiary alcohol

Question 20

What is the usual oxidation mixture used in the oxidation of alcohols?

Answer 20

Potassium dichromate (VI)

Question 21

Dehydration

Answer 21

Is any reaction in which a water molecule is removed from the starting material

Question 22

Dehydration of alcohols

Answer 22

And alcohol is heated under reflux in the presence of an acid catalyst such as concentrated sulphuric acid, or concentrated phosphoric acid. The product of the reaction is an alkene
Dehydration of an alcohol is an example of an elimination reaction

Question 23

Substitution reactions of alcohols

Answer 23

Alcohols react with hydrogen halides to form haloalkanes. When preparing a haloalkanes, alcohol is heated under reflux with sulphuric acid and a sodium halide the hydrogen bromide is formed in place
The hydrogen bromide formed reacts with the alcohol to produce the haloalkane

Question 24

Naming the haloalkanes

Answer 24

Haloalkanes are compounds containing the elements carbon, hydrogen, and at least one halogen. When naming them, a prefix is added to the name of the longest chain to indicate the identity of the halogen. When two or more halogens are present in the structure they are listed in alphabetical order

Question 25

Halogens

Answer 25

Fluorine, chlorine, bromine, iodine

Question 26

Prefixes used to represent the different halogens in haloalkanes

Answer 26

Fluoro chloro, bromo, iodo

Question 27

Reactivity of the haloalkanes

Answer 27

Haloalkanes have a carbon-halogen bond in their structure. Halogen atoms are more electronegative than carbon atoms. The electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom. The carbon-halogen bond is polar
In haloalkanes the carbon atom has a slightly positive charge and can attract species containing a lone pair of electrons. Species that donate a lone pair of electrons are known as nucleophiles
When a Haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction. The new compound is produced containing a different functional group. The reaction mechanism is nucleophilic substitution

Question 28

Nucleophile

Answer 28

An atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
Hydroxide ions, water molecules and ammonia molecules

Question 29

Nucleophilic Substitution in the haloalkanes

Answer 29

Primary haloalkanes undergo nucleophilic Substitution reactions with a variety of different nucleophiles to produce a wide range of different compounds
Substitution is a reaction in which one atom or group of atoms is replaced by another atom or group of atoms

Question 30

Hydrolysis

Answer 30

A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule. This results in the molecule being split into two products

Question 31

Hydrolysis of a haloalkane

Answer 31

The halogen atoms is replaced by an –OH group.

Question 32

Hydrolysis and carbon – halogen bond strength

Answer 32

In hydrolysis, the carbon – halogen bond is broken and the –OH group replaces the halogen in the haloalkane. The rate of hydrolysis depends upon the strength of the carbon – halogen bond in the haloalkane.
C – F bond is the strongest
C – I bond is the weakest

Question 33

Organohalogen compounds

Answer 33

Molecules that contain at least one halogen atom joined to a carbon chain.
Rarely found in nature and as they aren’t broken down naturally in the environment, they have become the focus of some concern

Question 34

Uses of Organohalogen compounds

Answer 34

General solvents
Dry cleaning solvents
Making polymers
Flame retardants 
Refrigerants 
Pesticides

Question 35

The ozone layer

Answer 35

Found at the outer edge of the stratosphere, at a height that varies from about 10 to 40km above the earth’s surface.
Only a tiny fraction of the gases making up the ozone layer is ozone, but that’s enough to absorb most of the biologically damaging UV radiation from the sun’s rays, allowing only a small amount to reach the earth’s surface.
In the stratosphere, ozone is continually being formed and broken down by the action of UV radiation
O2 + O <=> O3
Human activity, especially in the production and use of chlorofluorocarbons, has upset this delicate equilibrium

Question 36

CFCs and the ozone layer

Answer 36

CFCs are very stable because of the strength of the carbon-halogen bonds within their molecules.
In 1973 two chemists concluded that CFCs remain stable until they reach the stratosphere. Here the CFCs begin to break down, forming chlorine radicals, which are thought to catalyse the breakdown of the ozone layer

Question 37

How do CFCs deplete the ozone layer?

Answer 37

When in the stratosphere UV radiation provides sufficient energy to break a carbon-halogen bond in CFCs by homolytic fission to form radicals. The C-Cl bond has the lowest bond enthalpy so is the bond that breaks first
Photodissociation- CF2Cl2–> CF2Cl* + Cl*
Propagation step 1- Cl* + O3–> ClO* + O2
Propagation step 2- ClO* + O—> Cl* + O2
Overall equation- O3 + O—> 2O2

Question 38

Are CFCs responsible for all ozone depleting reactions?

Answer 38

No. Other radicals also catalyse the breakdown of ozone. Nitrogen oxide radicals are formed naturally during lightning strikes, and also as a result of aircraft travel in the stratosphere.
Propagation step 1- NO* + O3 —> NO2* + O2
Propagation step 2- NO2* + O—> NO* + O2
Overall equation- O3 + O —> 2O2

Question 39

Quickfit apparatus

Answer 39

Round bottom or pear shaped flask
Receiver
Screw-tap adaptor
Condenser
Still head

Question 40

Heating under reflux

Answer 40

Organic resources are heated to overcome the activation energy and increase the rate of reaction
It is a common procedure used to prepare an organic liquid without boiling off the solvent reactants or products.

Question 41

Heating under reflux apparatus

Answer 41

Round bottom or pear shaped flask
 Condenser
Rubber tubing
Stand and clamp
Heat source

Question 42

Distillation

Answer 42

Method used to separate a pure liquid from its impurities
The flask is heated and the mixture in the flask will start to boil. The different liquids in the mixture will have different boiling points. The liquid with the lowest boiling point is the most volatile and will boil first
The vapour moves out of the flask up into the other parts of the apparatus, leaving behind the less volatile components of the mixture. When the vapours reach the cold condenser, they condense and become liquid. Thus liquid then drips into the collecting flask

Question 43

Distillation apparatus

Answer 43

Round bottom or pear shaped flask 
Condenser
Rubber tubing
Heat source
Stand and clamp
Screw cap adaptor
Receiver adaptor
Still head
Thermometer

Question 44

Purifying organic products

Answer 44

When preparing samples of organic liquids water may be obtained along with the product. If this has happened you will see two liquid layers inside your collection flask, one the organic layer and one the aqueous or water layer
The two layers are separated using funnel. Ensure the tap of the separating funnel is closed, pour the mixture into the funnel, place a stopper in the top of the funnel, and invert to mix the contents
Allow the layers to settle. Add some water to see which layer increases in volume- this is the aqueous layer
Place a conical flask under the funnel, remove the stopper and open the tap until the whole of the lower layer has left the funnel
Place a second clinical flask under the funnel to collect the other layer

Question 45

Drying an organic product

Answer 45

There may be some water left in the organic product.
Add the organic liquid to a conical flask. Using a spatula, add some of the drying Agent to the liquid gently swirl
Place a stopper on the flask to prevent your product from evaporating away. Leave for 10mins
If the solid has all stuck together in a lump, there is still some water present. Add more drying agent until some solid is dispersed in the solution as a fine powder
Decant the liquid from the solid into another flask. If the liquid is dry it should be clear

Question 46

Redistillation

Answer 46

Sometimes organic liquids have boiling points that are relatively close together, so your prepared sample may still contain some organic impurities. The distillation apparatus is cleaned and dried and set up again so a second distillation can be carried out. This time, only collect the product with the boiling point of the compound you are trying to make. The narrower the boiling range, the purer the product.

Question 47

Mass spectra

Answer 47

Can be used to identify the molecular mass of an organic compound and to gain further information about its structure

Question 48

Molecular ions- mass spectrometry

Answer 48

When an organic compound is placed in the mass spectrometer, it loses an electron and forms a positive ion, the molecular ion
The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound

Question 49

Finding the Molecular mass from a mass spectrum

Answer 49

The molecular ion peak has to be located. It’s a clear peak at the highest m/z value on the right hand side of the mass spectrum.

Question 50

Fragmentation- mass spectrometry

Answer 50

In the mass spectrometer some molecular ions break down into smaller pieces known as fragments in a process called fragmentation. The other peaks in a mass spectrum are caused by fragment ions, formed from the breakdown of the molecular ion
The simplest fragmentation breaks a molecular ion into two species- a positively charged fragment ion and a radical. Any positive ions formed will be detected by the mass spectrometer but the uncharged radicals aren’t detected.

Question 51

Using fragmentation peaks to identify an organic molecule

Answer 51

The mass spectrum of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures. Mass spectra can therefore be used to help identify molecules.
So even though two molecules may have the same molecular mass and the same molecular ion peak, the fragment ions found in the spectrum may be different

Question 52

Infrared radiation and covalent bonds

Answer 52

Atoms in molecules are joined by covalent bonds. These bonds possess energy and vibrate naturally about a central point, the amount of vibration increasing with increasing temp. The atoms in molecules are therefore in constant motion. The bonds can absorb infrared radiation, which makes them bend or stretch more

Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond. The frequency values are very large, so chemists use a more conventional scale called wavenumber, which is proportional to wave frequency. The vibrations of most bonds are observed in the IR wavenumber range of 200cm-1 to 4000cm-1

Question 53

Stretch

Answer 53

One type of vibration. It’s a rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases

Question 54

Bend

Answer 54

One type of vibration. Results in a change in bond angle

Question 55

The amount that a bond stretches or bends depends on:

Answer 55

• The mass of the atoms in the bond- heavier atoms vibrate more slowly than lighter ones
• the strength of the bond- stronger bonds vibrate faster than weaker bonds

Question 56

Infrared radiation and atmospheric gases

Answer 56

Much of the sun’s visible and IR radiation is relatively unaffected by atmospheric gases. This radiation passes through the atmosphere to the earth’s surface, where most of it is absorbed. However, some is re-emitted from the earth’s surface in the form of longer-wavelength IR radiation
Water vapour, CO2 and methane (greenhouse gases) absorb this longer-wavelength IR radiation, because it has the same frequency as the natural frequency of their bonds. Eventually, the vibrating bonds in these molecules re-emit this energy as radiation that increase the temp of the atmosphere close to the earth’s surface, leading to global warming.

Question 57

Infrared spectroscopy and organic molecules

Answer 57

Organic chemists use infrared spectroscopy as a means of identifying the functional groups present in organic molecules.

1. The sample under investigation is placed inside an IR spectrometer.
2. A beam of IR radiation in the range 200-400 cm-1 is passed through the sample.
3. The molecule absorbs some of the IR frequencies, and the merging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample
4. The IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber.

Below 1500cm-1 there are a number of peaks in what is known as the fingerprint region of the spectrum. The fingerprint contains unique peaks which can be used to identify the particular molecule under investigation, either using computer software or by physically comparing the spectrum to booklets of published spectra

Question 58

O-H Group in alcohols wavenumber

Answer 58

3200-3600 cm-1

Question 59

C=O group in aldehydes, ketones and carboxylic acids wavenumber

Answer 59

1630-1820 cm-1

Question 60

COOH group in carboxylic acids wavenumber

Answer 60

C-O is 1000-1300
C=O is 1630-1820
O-H is 2500-3300

Question 61

Applications of infrared spectroscopy

Answer 61

Many pollutants can be identified by their IR spectral fingerprints. Remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide and hydrocarbons in busy town centres or by motorways to monitor localised pollution
IR-based breathalysers pass a beam of IR radiation through the captured breath in the sample chamber and detect the IR absorbance of the compounds in the breath. The characteristic bonds present in ethanol are detected. The more IR radiation absorbed, the higher the reading, and the more ethanol in the breath

Question 62

Typical sequence for identification

Answer 62

• Elemental analysis- use of % composition data to determine the empirical formula
• mass spectrometry- use of molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule
• infrared spectroscopy- use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule

Once you have both the empirical formula and molecular mass of a compound, you can determine the molecular formula of your unknown compound. By then using evidence from the infrared spectrum, it may be possible to identify an unknown compound