Organic Chemistry - Families 1 (Alkanes, Alkenes & Alkynes - The Hydrocarbons)

Give the name of the first 5 members of the alkanes. For each member named, give its molecular formula and draw its structural formula.

Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12

Ethene is an unsaturated compound. Describe 2 tests that can be performed on the ethene gas to show this including what would be observed.

1) Addition of bromine- test for unsaturation. Bromine water is added to a test tube containing ethene gas, the test tube is stoppered and shaken. The red bromine water turns colourless since ethene is unsaturated. 2)Addition of accidified potassium permanganate-testfor unsaturation Purple accidified potassium permanganate is added to a test tube containing ethene gas, the test tube is stoppered and shaken. The purple accidified potassium permanganate turns colourless since ethene is unsaturated.

Ethyne is an unsaturated compound. Descibe 2 tests that can be performed on the ethyne gas to show this, including what would be observed.

1) Addition of bromine- test for unsaturation. Bromine water is added to a test tube containing ethyne gas, the test tube is stoppered and shaken. The red bromine water turns colourless since ethyne is unsaturated. 2)Addition of accidified potassium permanganate-testfor unsaturation Purple accidified potassium permanganate is added to a test tube containing ethyne gas, the test tube is stoppered and shaken. The purple accidified potassium permanganate turns colourless since ethyne is unsaturated.

Give 2 safety precautions for the experiment to prepare ethyne.

1)Ethyne is explosive with air - keep any naked flame away from the apparatus. 2)Use a spatula to place calcium carbide into the flask - can be an irritant to the skin.

>What solid is used in the experiment to prepare ethyne? Describe the precise appearance of this solid. >Explain the white colour that forms as the reaction proceeds. >Explain why the conical flask gets warm as the reaction proceeds.

>The calcium carbide is a grey-black solid. >The white substance forms as a result of the production of calcium hydroxide [ Ca(OH)2 ]. >The flask gets warm because the reaction between calcium carbide and water is exothermic (gives out heat).

Comment on the geometry around the carbon atoms in alkene molecules.

> In alkenes , the carbon in the carbonyl group has planar geometry - bonded to two other elements by a single covalent bond and an oxygen by a double covalent bond (3 bond pairs of electrons, no lone pairs) >All other carbons, have tetrahedral geometry - bonded to 4 other elementd by single covalent bonds (4 bond pairs of electrons, no lone pairs)

Explain how only sigma bonding is found between the carbon atoms of alkanes.

>Alkanes are saturated compounds - contain only single bonds between the carbon atoms. >Single bonds are formed by a sigma bond i.e. the head on overlap between two atomic orbitals.

Explain why the first four members of the alkenes exist as gases at room temperature while higher members are liquids.

>Alkenes are non-polar molecules and have Van der Waals forces between their molecules. > Van der Waals are weak and do not require much energy to be broken - explains why the first four members (C1-C4) are gases at room temperature. > As the molecular mass of alkenes increases, the strength of the intermolecular forces increases and boiling point increases. This explains why higher members are liquids and gases.

Explain why alkenes are poorly water soluble and highly soluble in cyclohexane.

>Alkenes are non-polar molecules. >Therefore, they will not dissolve in water but will dissolve in non-solvents such as cyclohexane.

Explain how sigma bonding and pi bonding are found between the carbons in the alkenes.

>Alkenes are unsaturated compounds - contain a double bond between two of the carbon atoms. >Double bonds are formed by a sigma bond (i.e. the head on overlap of two atomic orbitals )and a pi bond (i.e. the sideways overlap of two p or d atomic orbitals)

Comment on the solubility of chloroalkanes.

>Chloroalkanes are insoluble in water, very soluble in non-polar solvents such as cycloalkanes such as cyclohexane. > Most of the molecule is non-polar.

A lighted splinter was placed in a test tube of ethene gas collected. A sample of limewater was then added to the products of the combustion. >What reaction occurs when ethene is combusted? >What reaction occurs when limewater is added to the products of this combustion?

>Ethene burns with a bright yellow flame. C2H4 + 3O2--------> 2CO2 + 2H2O >The carbon dioxide formed in the combustion turns the limewater milky. Ca(OH)2 + CO2-------> CaCO3 + H2O Lime water White Calcium Carbonate is formed

In the preparation of ethene , comment on the appearance of ethene gas collected.

>Ethene is a colourless gas with a sweet smell. >Ethene is insoluble in water but dissolves in non-polar solvents e.g. cyclohexane.

Give 3 safety precautions for this experiment- preparation of ethene.

>Ethene is falmmable -ensure the test tube stopper is airtight to avoid any ethene escaping. >Wear gloves when handling glass wool. >Raise the apparatus from the water before turning the bunsen burner off.

>Give the name of the first member of the alkynes. >Give its common name. >Give its molecular formula . >Give an industrial use of this alkyne.

>Ethyne >Common name - Acetylene >C2H2 >Ethyne gas is used in oxyacetylene welding and cutting - when ethyne is burned in oxygen it burns at extremely high temperatures (>3000 deg celcuis)

A lighted splinter was placed in a test tube of ethyne gas collected. >Describe how the ethyne burned >Account for the soot produced when combusting ethyne. >Compare the flame observed when combusting ethyne to combusting ethene. > Write a balanced chemical equation for the combustion of ethyne.

>Ethyne burns with a bright smoky flame with a lot of soot formed. >The soot is formed as a result of unburned carbon. >The flame observed when burning ethyne is brighter and smokier than when burning ethene. >C2H2 + 2 1/2 O2-------> 2CO2 + H2O

What is a sigma bond?

>Single bonds are formed by a sigma bond i.e. the head on overlap between two atomic orbitals.

Give 2 uses of chloroalkanes.

>Solvent for non-polar substances - used in dry cleaning. >Paint stripper (dichloromethane)

Alkanes are saturated compounds. Explain what this means.

A saturated compound has only single bonds between the atoms in the molecule.

>In the prepartion of ethyne, what substance is the ethyne gas passed through? >Explain why ethyne gas is passed through this substance. > Explain why these impurities may be present in the ethyne gas produced.

>The ethyne gas is passed through accidified copper sulfate. >The ethyne gas formed will not be pure.Passing it through accidified copper sulfate removes impurities. >Calcium carbide is often contaminated with calcium sulfate, calcium phosphide and calcium nitride..

In the preparation of ethene gas, what is the precise appearance of the solid placed in the test tube? Why is this powder heated gently and the ethanol itself is not heated?

>The solid Aluminium oxide is a white powder. >If the ethanol was heated directly it would change quickly to a vapour and pass over the aluminium oxide too quickly - not all of the ethanol would be converted into ethene.

>In the preparation of ethene gas in the lab why is it important that the apparatus is raised from the water before heat source is turned off ? >What is the danger involved with turning off the heat source before removing the apparatus?

>To avoid "suck-back" of water up the delivery tube. >Cold water being sucked back up into the hot test tube could cause it to crack.

What is a homologous series?

A homologous series is a family of organic compounds with the same general formula, similar chemical properties with successive members differing by CH2.

What is a pi bond?

A pi bond is formed by the sideways overlap of two p or d orbitals.

Explain why the alkynes are unsaturated compounds.

Akynes are unsaturated compounds containing triple bonds.

Write a balanced chemical equation for the reaction involved when preparing ethene. How is the ethanol kept in the test tube? What solid is placed in the test tube? Give 2 functions of this solid .

Al2O3 C2H5OH ------> C2H4 + H2O >Glass wool is soaked in ethanol and kept in the test tube. >The solid placed in the test tube is Aluminium oxide (Al2O3) > Aluminium oxide i) acts as a dehydrating agent. ii) is a catalyst for the reaction.

Name the 8 homologous series studied in Leaving Cert Chemistry.

Alkanes Alkenes Alkynes Alcohols Aldehydes Ketones Carboxylic acids Esters

Comment on the solubilities of alkanes.

Alkanes are non-polar molecules. Therefore, they will not dissolve in water but will dissolve in non-polar solvents such as cyclohexane.

Explain why the first four members of the alkanes exist as gases at room temperature.

Alkanes have Van der Waals forces between their molecules. Van der Waals forces are weak and do not require much energy to be broken.This explains why the first four members of alkanes are gases at room temperature.

The alkenes are unsaturated compounds. Explain what this means.

An unsaturated compound has double or triple bonds between some of the carbon atoms in the molecule.

What is the molecular formula of cyclohexane?

C6H12

Write a balanced chemical equation for the reaction involves when preparing ethyne.

CaC2 + 2H2O --------> Ca (OH)2 + C2H2

What are chloroalkanes?

Chloroalkanes are compounds in which one or more of the hydrogen atoms in an alkane molecule have been replaced by chlorine atoms.

The boiling point of butane is -1 deg Celcius while the boiling point of chlorobutane is 78.5 deg Celcius. ,Account for the lower boiling point of butane.

Cloroalkanes have higher boiling points than their corresponding alkanes due to having a greater molecular mass.

What is the general formula of alkenes?

CnH2n

What is the general formula of alkanes?

CnH2n+2

What is the general formula of the alkynes?

CnH2n+2

Give an industrial use of ethene.

Ethene generators are commercially used to speed up ripening of fruit.

Give the names and molecular formulas of the first three members of the alkenes.

Ethene, C2H4 Propene C3H6 Butene C4H8

Comment on the geometry around the carbon atoms in alkane molecules.

In alkanes, every carbon atom has tetrahedral geometry - bonded to 4 other elements by single covalent bonds ( 4 bond pairs of electrons, no lone pairs)

Write a balanced chemical equation for the reaction between ethene gas and bromine.

H H Br Br C = C + Br-Br--------> H- C - C-H H H H H Ethene Bromine 1,2-dibromoethane

What are hydrocarbons?

Hydrocarbons are compounds that contain the elements carbon and hydrogen only.

In the prepartion of ethyne, name 3 impurities that could be present in the ethyne gas produced and explain why these impurities may be present.

Hydrogen sulfide Phosphine Ammonia

What is the chemical name and chemical formula of limewater?

Lime water Calcium hydroxide - chemical name Ca(OH)2 - chemical formula

What is organic chemistry

Organic chemistry is the study of the compounds of carbon.

What are structural isomers?

Structural isomers are compounds that have the same molecular formula but different structural formulas.

In the preparation of ethene in the lab, why are the first few test tubes of gas collected discarded?

The first few test tubes consist mostly of displaced air and not ethene gas.

Explain why the first few test tubes of gas collected discarded. >Describe the appearance of ethyne gas collected.

The first few test tubes consist mostly of displaced air and not ethyne gas.