The Alkane - a homologous series



The Alkane - a homologous series

I). Introduction to Organic Chemistry

Organic chemistry is a study of carbon chemistry.

Organic compound is applied to all carbon-containing compounds and usually comes from petroleum. However, the oxides of carbon and carbonates are described as inorganic compounds.

Carbon - a Unique element

Carbon combines with many other elements, especially hydrogen and oxygen to form millions of compounds because carbon have the following properties:

1. It tends to share the four electrons with other elements to form four covalent bonds. The four covalent bonds are actually arranged in a tetrahedral shape.

C

2. Carbon can form stable straight chain, branched chain and ring structure.

Straight chain branched chain ring chain

3. Carbon can form strong single, double and triple bonds with another carbon atom

single bond double bond triple bond

A). Representing Molecular Structure of Organic Compounds

Molecular formula - formula just showing the number of different atoms in the molecule.

Structural formula - formula showing how the atoms are joined to one another in the molecule.

|Molecular formula |C4H10 |C4H10 |

|Structural | | |

|formula | | |

| | | |

|Name |butane |2-methylpropane |

|Molecular formula |C3H6Cl2 |C3H6Cl2 |C3H6Cl2 |

|Structural | | | |

|formula | | | |

| | | | |

|Name |1,1-dichloropropane |1,2-dichloropropane |2,2-dichloropropane |

Condensed Structural Formula - structural formula written in a condensed form

|Structural formula |Condensed structural formula |

|[pic] | |

|[pic] | |

|[pic] | |

|[pic] | |

|[pic] | |

|[pic] | |

B). Molecular models

The structure of organic compounds are actually three-dimensional.

The structural formulae provide only two-dimensional representations

The actual molecular shapes can be represented by

1. ball - and - stick model

2. space-filling model

C). Hydrocarbons

Organic compounds that contain only carbon and hydrogen atoms are called ___________

Hydrocarbons with carbon atoms forming only carbon-carbon single bond (i.e. C(C) are said to __________[pic]

Hydrocarbons containing one or more carbon-carbon multiple bonds (that is C(C or C(C) are said to be _____________. [pic]

Do Classwork in P.22

II). The Alkanes

A). Introduction

Petroleum and natural gas are mainly mixtures of a type of hydrocarbon called alkanes.

|Name |Molecular formula |Structural formula |

|methane | | |

|ethane | | |

|propane | | |

|butane | | |

|pentane | | |

Can you construct a general molecular formula of the alkane molecules?

B). Homologous Series

Alkanes can be represented by the general formula ___________.

The Alkanes is a group of organic compounds called a homologous series.

Members in homologous series have the following properties.

1. with the same general formula

2. successive members differing by a _________ unit

3. show a graduation in physical properties.

4. show similarity in chemical properties.

C). Physical properties of alkanes

|Number of |Name |Molecular |Melting |Boiling point |Density |State at R.T.P. |

|carbon atoms | |formula |point ((C) |((C) | | |

|1 |methane |CH4 |-183 |-162 |- |gas |

|2 |ethane |C2H6 |-172 |-89 |- |gas |

|3 |propane |C3H8 |-188 |-42 |- |gas |

|4 |butane |C4H10 |-135 |-0.5 |- |gas |

|5 |pentane |C5H12 |-130 |36 |0.62 |liquid |

|6 |hexane |C6H14 |-95 |69 |0.66 |liquid |

|7 |heptane |C7H16 |-91 |98 |0.68 |liquid |

The C1 to C4 members of alkane are gases, the C5 to C17 are colourless oily liquids while higher members are solids.

As the number of carbon atoms in the alkane chain ________, the melting and boiling points _______.

It is because as the molecules become larger and heavier, the forces of attraction (________________ forces ) among the molecules also ________. As a result, _______ energy will be needed to separate the molecules during melting and boiling.

[pic]

D). Chemical Properties of alkanes

Alkanes are chemically unreactive because their carbon atoms form only carbon-carbon single bonds (i.e. they are saturated.) They do not react with acids, alkalis, oxdizing agents or reducing agents.

1). Combustion

a).Alkane burns completely in excess air or oxygen to give ____________ and _________ as the only products.

Equation: C3H8 + O2 (

C7H16 + O2 (

C8H18 + O2 (

The presence of carbon and hydrogen in an alkane can be investigated using the apparatus shown below:

[pic]

13. ________ gas (from carbon in alkane) turns the lime water ________.

14. ________ (from hydrogen in alkane) turns dry blue cobalt(II) chloride paper ________.

15. A large amount of heat is produced in this exothermic reaction. So alkanes are mainly used as fuels.

b). If insufficient oxygen is provided for burning, methane and other alkanes will burn with a _____ flame and produce ______. This means the combustion is _________.

2). Reaction with halogens (substitution reaction)

When methane and bromine are mixed in the dark, no reaction occurs.

In diffused sunlight (ultraviolet light or heating), alkane react slowly with bromine to give a mixture of halogenalkanes

equations:

(chain reactions)

The observation is :The red colour of bromine slowly disappears.

The hydrogen atoms are successively replaced or substituted by bromine atoms. This is an example of substitution reaction.

Substitution reaction is a reaction in which an atom (or a group of atoms) in a molecule is replaced by another atom (or a group of atoms).

When chlorine is used instead of bromine, a similar and ______ reaction takes places.

Iodine has almost no reaction with methane.

The reaction of methane with halogens indicates the reactivity of the halogens _______ down the group.

III). Naming the alkanes (IUPAC Names)

A). Straight-chain Alkanes

24. All the names of alkanes end with the suffix `-ane`

25. The number of carbon atoms are indicated by the prefix:

|Number of carbon atoms |Molecular formula |Prefix(the beginning of |Suffix (the end of the|Name |

| | |the word) |word) | |

|1 | |meth- | | |

|2 | |eth- | | |

|3 | |prop- | | |

|4 | |but- | | |

|5 | |pent- |-ane | |

|6 | |hex- | | |

|7 | |hept- | | |

|8 | |oct- | | |

|9 | |non- | | |

|10 | |dec | | |

B). Branched-chain alkane

The IUPAC name for a branched-chain alkane consists of 2 parts:

side groups

main carbon chain

The IUPAC rules for naming branched-chain alkanes are as follows:

1. Locate the longest continous chain of carbon atoms.

examples

[pic][pic][pic]

______ chain ______ chain _______ chain

2. Number the longest continous chain, starting with the carbon atom at the end nearest the point at which branch chains are attached.

Examples:

[pic] [pic]

3. Specify the position number of the carbon atoms to which each branch chain or side group is attached. Name each side group using the prefix shown below

|branch chain |CH3( |CH3CH2( |CH3CH2CH2( |F( |Cl( |Br( |I( |

|/side chain | | | | | | | |

|prefix |methyl |ethyl |propyl |fluoro |chloro |bromo |iodo |

Example:

[pic]______________

4. If two or more identical groups are attached to the carbon chain, specify this by using the following prefixes:

|no. of identical groups |two identical group |three identical group |four identical group |

|prefix |di |tri |tetra |

Example:

[pic][pic][pic]

___________________ ____________________ ___________________

5. If two or more different side groups are attached to the main carbon chain, list them in alphabetical order.

A comma (,) is written between two numbers A hyphen (-) is written between a number and an alphabet.

Example:

[pic][pic]

______________________ ________________________

Classwork: Give the IUPAC names to the following compounds.

a). b).

[pic] [pic]

__________________________ ___________________________

c). d). e).

[pic] [pic] [pic]

_____________________ ______________________ ________________________

IV). Writing Structural Formulae from IUPAC Names

If the IUPAC name of a compound, its structural formula can be written, following the steps given below.

1. Write down the main carbon chain.

2. Number the main chain.

3. Add the side groups to correct places in the main chain.

4. Complete the structural formula by adding the correct number of hydrogen atoms. Check that each carbon atom has 4 bonds attached to it.

Examples:

a). 3-ethyl-2-methylhexane b). bromotrichloromethane c). 2-bromo-1-chlorobutane

Classwork

a). 2-methylpentane b). 2,2-dimethylpropane c).2,2,3-trimethylbutane d). 1,1,1-trichloroethane

V). The Organic Compounds with other Functional Groups

Consider the compounds ethane [pic]and ethanol [pic]. They have similar structures but they have very different properties.

| |Ethane |Ethanol |

|State at R.T.P. |Gas |Liquid |

|Reaction with sodium |No reaction |Reacts to give hydrogen gas |

| | |CH3CH2OH + Na ( CH3CH2O Na + H2 |

Ethane is saturated compound which usually does not react with metal. But the (-O-H) hydroxyl group in ethanol make it react with sodium. The (-O-H) is called a functional group

27. A family of compounds containing the same functional group form a homologous series.

Some homologous series with the functional group each contains are shown below

|Name of |Functional group|suffix in Naming |Examples |

|homologous series | | | |

|Alkenes |[pic] | | |

|Akynes |[pic] | | |

|Alkanols |-OH | | |

|Alkanals |[pic] | | |

|Alkanoic acids |[pic] | | |

|Amines |[pic] | | |

Examples:

a). b). 3-methylpent-1-ene

[pic]

__________________________

c). d). 3-methlybutan-2-ol

[pic]

__________________________

e). f). 2-methylpropanoic acid

[pic]

__________________________

VI). Structural Isomerism

28. Isomers are compounds with the same molecular formula but different structural formulae.

29. Structural Isomers have the same chemical properties because they contain the same functional group.

30. Structural Isomers have different physical properties. (e.g. m.p., b.p. density.) This is because the structures and shapes of the molecules are different. The intermolecular forces between the molecules are affected.

Examples:

1. C4H10

2. C4H8

3. C4H9OH

4. C3H7COOH

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