Chapter 21 - Organic chemistry

Organic chemistry

the study of compounds of carbon

Hydrocarbons

are defined as compounds containing only carbon and hydrogen.

3 types of hydrocarbons

alkanes, alkenes, alkynes

saturated compound

A saturated compound is one in which there are only single bonds between the atoms in the molecule.​

unsaturated

An unsaturated compound is one that contains one or more double or triple bonds between the atoms in the molecule.​

​

Alkanes

CnH2n+2 i.e if n = 5 or then C5H(2)(5) + 2 = C5H12 or pentane

List of Alkanes

methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane

Need to knwo alkanes

Methand, ethane Propane, Butane Hexane

Alkanes

Consist of only single carbon-carbon bonds.​

Compounds with only single carbon-carbon bonds are referred to as saturated.​

All carbons are tetrahedral.​

Boiling points: Low due to (Van Der Waals) but increase with molecular size.​

The Alkane family have many similar properties, so we call them a homologous series.​

Application - Fuel (methane)​

Homologous series​

- A series of compounds of similar chemical properties

- SHowing gradation in physical properties

- Having general formula for its memebers

- Each member having a similar formula

_ each member having a similar method of preperation

Structural isomers

Compounds with the same molecular formula but different structural formula

What is used to help name alkanes

Side - chains grouos of atoms attatched to the main carbon chain

Alkyle group - an alkane atom from which a hydrogen atom has been removed

1. Methyl - CH3

2. ethyl group - C2H5

3.Propyl group - C3H7

What happens when the same alkyle group is present more than once

- Present twice - di

- Present thrice - tri

How to name an alkane

- Check how manycarbons in the main chaine

- Check for alkyl groups - is it Ch3 or C2H5 or C3h7

- Check what group of carbons the side chain is attatched to

How to number alkane chains

Identify the side which is closer to the side chain and then number it from there

How to name alkane chain with two different substients

First state at what carbon the first substient is at (3) and then (0f ot occurs more than one time state di or tri) then state the substients name

- For different substients individually do it

Draw the full structural formula and give the systematic name of the following?​

CH3CH(CH3)CH3

The bracket is where ths ide chain is

Alkenes

Hydrocarbons with one or more carbon-carbon double bonds

how to name alkenes

- With alkenes change the ending to ene

- And the formula fpr the H in ch is CnH2n so C4 = C4h8

alkenes list (Note: On this course you need to only be concerned on how to draw the first three alkenes when giving molecular formula).​

Ethene - C2H4, Propene - C3H6, Butene - C4H8, Pentene - C5H10 etc. Methane cannot be an alkene

alkene properties

Consist of one double carbon-carbon bond. Classified as unsaturated.​

Boiling points: Low due to (Van Der Waals) but increase with molecular size.​

The C=C bond is planar.​

Application: Ethene is used to ripen bananas.​

How to name alkenes

Identify longest chain (must contain double bond) & identify prefix based on no of carbons.

Add -ene.

Insert number before -ene to indicate which carbon the double bond belongs to (choose smallest number).

Name & state location of any branches (same rules as for alkanes).

1. State where the branch chane is and identify the tyoe or whether there is on two or three

2. Idemtify the longest chain and then state how many cabrons i.e 6 = hexene

3. Identify where the bonding occurs or where it starts if at C4 then its hex 4 ene

start numbering based on the side which has the bond

Alkynes​ properties

Consist of a triple bond.​

Highly unsaturated due to triple bond​

Ethyne (C2H2) is the only alkyne you need to study.​

Application: Welding and cutting (Ethyne "acetylene torches").​

Prepare ethane and examine its properties

- Using aluminum oxide this acts a dehydrating agent and is a catalyst for the reaction it acts as a dehydrating agent and also a catalyst for a reaction

Prepare ethane and examine its properties - Key steos

- Ethanol is poured into a boiling tube

- Glass wool is placed in this tube and

- The glass wool soaks up the ethanol and holds its in place at the end of the horizontal tube

- A small heap of aliminium oxide is placed in the middle of the tube half way along

- It is gently burned using the bunses burner

Prepare ethane and examine its properties - Observations

- Bubbles are formed in the test tube because ethano,changes into vapour as the aliminium oxide is heated. The ethanol changes into vapour and passes over the aliminium oxide

Prepare ethane and examine its properties - After experiment

5 test tubes of this gas are collected and a stopper is placed on each test tube

- The retots stand is loosened and raises up the apparatus

Suckback

-Occurs when delivery tube is still in water and removed Cold water is sucked along a delivery tube into the glass reaction vessel causing it to crack. This

happens if the source of heat is removed when the delivery tube is still in the water. A partial

vacuum occurs in the reaction vessel due to cooling of the gases in the closed tube,

What does the first tube contain

The first tube usually contains air displaced by the apparatus

Tests on ethene - Physical properties

- Ethene is a colourless gas with a sweet smell

- It is insoluble in water but soluble in chlorolm

Tests on ethene - Combustion

- Stopper removed from one of the test tubes using a lighted wax taper a light is applied to the mouth of the tube

- When gas has stopped burning some limewater is added to the test tube

- The gas burns with a yellow luminous flame - the flame can be described as slightly smokey

Tests on ethene - Adding Bromine

- Bromine is added to ethene

- Then the bromibe red water turns colourless it test the unsaturation of ethenses as ethenes and alkenes have a double bond

Test on ethene - Permangenate

Permangenate is added to the test tube and it turns from purple to colourless this is because it shows that ethane is a unsaturated compound

Alkynes

Change the ane to yne

- And is written in the formula as CnH2n-2

Need to know alkyne

ethyne (colourless and sweet smell)

To prepare ethayne andd examine its properties

Prepared using calcium dicarbide

To prepare ethayne - safety

Since ethyne is a hughly flammable gas no flame should be placed near the gas

To prepare ethayne - Procedure

- Calcium carbude is placed using a spatula in the buchner flask with a water dropper funnel on top

- a delivery tube is connected to a breaker containing acified copper sulfate to remove impurities such as hydrogen sulfide

To prepare ethyne observation

- Calcium dicarbride is a black grey solid

- When the water is ddropped into the reaaction flask ut starts tunring a milky white colour because calcium hydroxide is forming

- Bubbles of gas collect in the glass jar

To prepare bromine - what was observed when a few drops of dilute bromine was added to to the solution

A colour chnage from brown to colourless

What was observed when a few drops of acidified dilute potassium mangenate

Turns from purple tp colourless

Describe a est u could carry out to show that ethyne is an unsaturated compound

Adding acidified pottasium permangenate

Describe how to confirm that ethyne gas is flammable

Insert the lightenning tape into the tube it shiwcases that ethane is flammabke

- A luminous smoky flam is conceived and black soot is formed

What gas burns with a cleaner flame

Ethene because it does not have soot

Large scale use of ethyne

# wielding and cutting

.​Structural Isomers​

Structural isomers are compounds with the same molecular formula but different structural formulas.​

Structural Isomers​ - how to draw

Alkanes - Remove a CH3 (methyl group) from end of a molecule and place in the middle.​

Alkenes - Place the double bond on a different carbon.​

- Remove a CH3 (methyl group) from end and place in the middle i.e. C4H8​

Crude oil

(petroleum) is a fossil fuel that has been obtained from under the sea and hasn't been treated yet.​ Crude oil needs to be processed to be used efficiently as a fuel.

Fractional Distillation

Is used to separate crude oil into a number of useful parts. This process involves heating the crude oil and separating the various parts based on their boiling point.​

Smaller hydrocarbons:

Low boiling points - separate at the top fractions.​

Larger hydrocarbons:

High boiling points - separate at the bottom fractions.

Refinery gases:

methane, ethane, propane and butane.

Mercaptans

(sulphur compound) are added for safety to provide a smell so that leaks can be detected.​

LPG

Liquefied Petroleum Gas (LPG): Propane and Butane.

Fractional distillation diagram and how it works

1, Hot vaporised crude oil enters the bottom of the column.​

The column has a high temperature at the bottom and a low temperature at the top.​

Larger molecules have higher boiling points than smaller molecules.​

Larger molecules condense and are collected near the bottom of the column.​

Smaller molecules condense and are collected further up the column where it is cooler.

Background how petrol powers a car​

Petrol is pumped into engine, vaporised and mixed with air.​

Mixture is compressed by a piston and ignited by the spark plug.​

On combustion the gases produced expand and cause the piston to move (kinetic energy produced).​

When a petrol engine uses a fuel with a low octane number the fuel can auto-ignite which can cause.​

Damage to car​

Loss of power to the car.​

How octane number of fuel measured

The octane number of a fuel is a measure of the tendency of the fuel to resist knocking.​

Auto-ignition

Auto-ignition is premature ignition (explosion) of the petrol-air mixture before spark is produced.​(knocking)

Higher octane number

The higher the octane number the better the fuel.​

Two reference hydrocarbons in relation to octane number.​

2,2,4-trimethylpentane (iso-octane) : 100 (83:17 reference mixture)​

Heptane: 0​

Factors affecting Octane number​

Length of chaine

degree of branching the more branching a hydrocarbon has the higher its octane number

straight chain or cyclic nature

lead (no longer used as it is a cata,lyctic converter pouson)

increasing the octane number.​

Isomerisation - The changing of long straight chained alkane into its isomers - more branched.​

​2. Catalytic cracking - Splitting long chained hydrocarbons into short chain molecules using a catalyst. Note: An alkene is always produced.​

Catalyctic cracking

- catalytic cracking involves heating the hydrocarbon molecules to around 600 - 700°C to vaporise them

- the vapours then pass over a hot powdered catalyst of aluminium oxide or silica

- this process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions

- the long-chain alkane molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes

Increasing the octane number.​

3. Dehydrocyclisation: Forming of ring compounds, hydrogen gas is always given off.​

4. Adding oxygenates - Adding methanol, MTBE (Methyl Tertiary Butyl Ether) and ethanol are added to petrol.​

Note: Adding oxygenates has an advantage of less pollution.​

​

Two methods to produce hydrogen.

Steam reforming of natural gas​

Electrolysis of water.

Steam reforming of natural gas.

Methane + Steam = Hydrogen + carbon monoxide​

CH4 + H20 —> 3H2 + COMethane + Steam = Hydrogen + carbon monoxide​

CH4 + H20 —> 3H2 + CO

2. Electrolysis of water

Water + Electricity = Hydrogen +Oxygen​

H2O —> H2 + ½ O2

Advantages of hydrogen

- Produces a lot of energy

- burns cleaner than hydrocarbons

disadvantage

Explosive very little green hydrogen left

Uses of hydrogen

• use to produce ammonia

• Use to hydrogenate vegetable oils to produce margarine

• Burned as a fuel because it is environmentally friendly it only waste product is water

Exothermic

An exothermic reaction is one that releases heat energy energy exists the system it feels hot

Endothermic

Reaction is one that absorbs heat and energy

Hear of reaction

is the heat change when the number of moles of the reactants indicated in the balanced chemical equation for the reaction to react completely

If H value is negative

If the h value is negative the reaction is exothermic and it gets hotter

If the h value is positive

If the h value is positive the reaction is endothermic and gets cooler

Energy profile diagrams

Energy profile diagrams offer us a way of visualising the changes in energy during a reaction

Exothermic reaction

Endothermic reaction

Heat combustion

The heat change when one mole of a substance is burned in completely in excess oxygen

How can the heat of combustion of a substance can be experimentally measured b

The heat of combustion of a substance can be experimentally measured using a Bomb Calorimeter

The steps of using Bomb Calorimeter

• the sample to be tested is placed into a metal bomb along with pressurised oxygen gas

• Ignition wires ignite the sample

• The burning sample heats the water up

• The stirrer ensures the heat is evenly spread

• Then the thermometer reads the increase in the water temperature

The heat of combustion formula

Q = mc (t)

Q = heat energy

M = mass is f water (g)

C = specific heat capacity

Triangle + t = temperature change

Kilogram calorific value

The kilogram calorific value of a fuel is the heat energy produced when 1 kg of the fuel is burned completely

Writing balanced equations for heat of combustions

All hydrocarbons need oxygen to clmvusbahd the products formed will always be carbon dioxides and water

Writing balanced equations for heat and combustions

Fuel + O2 = Co2 + H2O

Balanced equation for propane

C3H8 + O2 - Co2 + H2O

How to balance equstions

Balance the c then the h then the o

Bond energy - where does h come from

H comes from the fact that in a reaction covalent bonds are broken and new ones are formed breaking bonds required energy making bonds releases energy

By calculating how much energy is needed to bank the bonds and how much energy is given off by making the new bonds we can calculate h

Bond energy

The average energy required to break one moles of a particular covalent bond and to separate the neutral atoms completely from eachother

Heat of neutralisation

The heat change when one moles of H + ions from an acid reacts with one mole of OH ions from a base

The heat of formation

The best of

Hess Law

tates that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. (Enthalpy is the heat energy of a whole system

how to do the calculations

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