Types of Reactions in Organic Chemistry - Chp 23

Mechanism of the monochlorination of methane

Substitution Reaction

Mechanism of the monochlorination of ethane

Substitution reaction

Addition reaction of Ethene with Bromine

Addition reaction of Ethene with Chlorine

Addition reaction of Ethene with Hydrogen chloride

Synthesis of PVC

Should be chlorines in the PVC structure

Subsitution reaction

a chemical reaction in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms

Mechanism

of a reaction is the detailes step-by-step description of how the overall reaction occurs

Chain reaction

is a reaction that continues on and on because a product from one step of the reaction is a reactant for another step of the reaction

Addition reaction

one in which two or more molecules react together to form a single molecule

Polymers

long chain molecules made by joining together many small molecules

Repeating

unit of a polymer is that part of the polymer whose repetition produces the complete polymer chain except for the end groups

Elimination Reaction

is one in which a small molecule is removed from a larger molecule to leave a double bond in the larger molecule

Redox reaction

occurs whenever there is a transfer of electrons from one chemical species to another

Give an example of a non-flammable organic compound:

a fully halogenated alkane

• such as bromochlorodifluoromethane

Why are fully halogenated alkanes no longer being used?

negative impact on the environment

Name 2 common oxidising agents:

Potassium permanganate (VII) - KMnO₄

Sodium dichromate (VI) - Na₂Cr₂O₇

- when acidified using conc. sulfuric acid they are strong oxidising agents, and are themselves reduced

Describe the colour of acidified KMnO₄ in its different oxidation states:

Purple - contains the MnO₄^- ion (Mn - ON of +7)

Brown - intermediate oxidation state (Mn - ON of +4)

Colourless - final oxidation state (Mn - ON of +2)

Describe the colour of acidified Na₂Cr₂O₇ in its different oxidation states:

Orange - contains the Cr₂O₇^-2 ion (Cr - ON of +6)

Green - final oxidation state (Cr - ON of +3)

What are primary alcohols oxidised to?

Primary alcohol → Aldehyde → Carboxylic Acid

What are secondary alcohols oxidised to?

Secondary Alcohol → Ketones

Write the reaction of ethanol with acidified sodium dichromate (VII)

Describe the reaction for the preparation of ethanal:

1. Set up apparatus for distillation. 

2. Place water and anti bumping granules in the pear shaped flask.

3. Whilst keeping the pear shaped flask under cold water add conc. H₂SO₄

4. Into the dropping funnel add sodium dichromate, water and ethanol. (Solution will be orange.)

5. Heat the acid until it is just boiling and then remove heat source (reaction is exothermic so temperature will be maintained).

6. Slowly add the dichromate/ethanol mixture at a rate that keeps the acid bubbling.

7. Distil as the ethanal is formed, keeping the distillate in an ice bath since ethanal is volatile.

8. To purify redistill the distillate, collecting the fraction that distils across at 20-23 ^oC.

What is the limiting reagent in the experiment for the preparation of ethanal?

• the oxidising agent is the limiting reagent

• the alcohol is in excess

Write the half equations for the oxidation of ethanol to ethanal

Draw the oxidation of ethanol to a carboxylic acid

Describe the reaction for the preparation of ethanoic acid

1. Set up apparatus for reflux

2. Add sodium dichromate and dilute sulfuric acid into the round bottomed flask. Add some conc. H₂SO₄ whilst keeping the flask cold.

3. Place some ethanol and water in the dropping funnel and slowly add the entire contents to the round bottomed flask.

4. Remove the dropping funnel and when the reaction has subsided reflux the mixture for 20 – 30 minutes using a water bath.

5. Rearrange for distillation (without a water bath, direct heating required due to temperature rising above 100 ^oC)

The bold are precautions to prevent excessive heat production

Write the half equations for the oxidation of ethanol to ethanoic acid

Organic Reaction Scheme

What is esterification?

formation of an ester and water from the reaction between an alcohol and carboxylic acid

What is hydrolysis?

the formation of an alcohol and carboxylic acid from an ester and water

Why can alcohols behave as acids?

• the polarity in the O-H bond makes it easier for the hydrogen atom to break off as a H⁺ ion

Describe how alcohols behave as an acid?

• they are very weak acids, 100 times weaker than water

• they will react with very reactive metals, such as the alkali metals

• they will NOT react with bases such as carbonates or hydroxides

Describe what happen when sodium is added to ethanol?

hydrogen gas is generated

Write an equation for the reaction between ethanol and sodium:

Describe what happens when ethanol is added to some sodium carbonate/hydroxide? 

nothing, as ethanol is too weak and acid to react 

What are the conditions necessary for a carboxylic acid to be reduced to the corresponding aldehyde and alcohol?

• in the presence of hydrogen (reactant) and nickel (catalyst)

Why can carboxylic acids behave as acids?

Inductive effect - the slightly positive carbon atom in the COH part of the molecule, draws electrons away from the hydrogen atom, so the hydrgoen of the hydroxyl group is lost readily

Stability of the carboxylate ion - when the carboxyl group loses a proton it forms the negative carboxylate ion, forming a resonance hybrid structure where the negative charge is spread over three atoms giving it extra stability, thus allowing it to lose protons readily

Describe the properties of ethanoic acid:

• vinegar odour

• changes universal indicator paper from green to red, as it is acidic

How can you distinguish between ethanoic acid and ethanol?

add magnesium metal, ethanol will not react with it as it is a weak acid and magnesium is not a reactive enough metal

Paper Chromatography - principle

Difference components of the mixture have different interactions with the mobile phase and stationary phase, so the different components of the mixture will travel different distances along the paper, separating the components of the mixture

Paper Chromatography - processes

1. add solvent to a chromatography tank

2. apply spot of mixture to the chromatography paper

3. dry

4. place in chromatography tank so that sport is just above solvent

5. components of mixture separate out as the mobile phase moves up through the paper

Paper Chromatography - use

• separate a mixture of colours/dyes

Gas Chromatography - principle

Different components of the mixture have different interactions with the stationary phase and mobile phase, thus the different components will travel at different speeds along the column separating the components of the mixture

stationary phase - liquid supported on a porous bed inside a long coiled column

mobile phase - inert gas

Gas Chromatography - processes

1. injection

2. transport of the sample along the column

3. separation in the column

4. detection

Gas Chromatography - use

• measure the level of alcohol in blood

• carry out drug tests on athletes

Mass Spectrometry

GC-MS Chromatography + applications

• is a method that combines the features of GL chromatography and mass spectrometry to identrify different substances within a test sample 

applications 

• drug detection 

• environmental analysis

• identification of unknown samples

High Performance Liquid Chromatorgaphy (HPLC) - principle

different components of the mixture have different tendencies to absorb onto very fine particles of a solid in the HPLC column and solvent that is pumped under pressure through columns so the different components will travel different speeds along the column thus separating the components of mixture 

High Performance Liquid Chromatography - processes

1. injection

2. transport of the sample along the column

3. separation in the column

4. detection

High Performance Liquid Chromatography - use

• identify the presence of growth promoters in meat/vitamins in food

• identify the presence of drugs

Infra Red Absorption Spectrometry - principle

molecules of a substance absorbs infra red light of different frequencies, and does so by vibrations of the bonds in the molecules, the combination of frequencies absorbed is particular to the molecules of that substance

Infra Red Absorption Spectrometry - processes

1. infra red radiation passes through the sample

2. the sample absorbs IR radiation at specific wavelengths which are detected

3. absorption spectrum is produced

Infra Red Absorption Spectrometry - use

• qualitative determination of compounds in plastics or drugs

• identifies compounds, but not the concentration

Ultraviolet Absorption Spectrometry - principle

absorption of ultraviolet radiation by molecules results in the promotion of electrons from their ground state energy levels to higher energy levels, and the absorbance is directly proportional to concentration

Ultraviolet Absorption Spectrometry - process

1. ultraviolet light is passed through the sample and a blank

2. the sample absorbs ultraviolet radation at a specific wavelengths which are detected 

3. absorption spectrum is produced

Ultraviolet Absorption Spectrometry - use

• quantitative determination of organic compounds such as drug metabolites/plant pigments

• gives concentration, but cannot identify usually

Atomic Absorption spectrometry - principle

atoms in the ground state absorb light of a particular wavelength characteristic of the element, and absorbance is directly proportional to concentration

Atomic Absorption spectrometry - process

1. sample solution is sprayed into the flame and the sample element is converted into atoms in the element

2. ground state atoms absorb radiation from a source made from the element

3. absorbance is measured

Atomic Absorption spectrometry - use

• detection and measuring concentration of heavy metals

• detection and measuring concentration of water pollutants

Colorimetry - principle

1. if a solution is coloured then the intensity of the colour is proportional to the concentration

2. the percentage of light absorbed by the coloured solution in the colorimeter is proportional to the concentration

Colorimetry - process

1. light of a particular wavelength is passed through a number of samples of known concentration 

2. a graph of absorbance against concentration is plotted 

3. the absorbance of the unknown is noted and using the graph the concentration of the unknown can be found

Colorimetry - use

• determination of the concentration of lead in water/fertilisers

• determination of the concentration of free chlorine in pool water