Topic 2: My version

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Name of Homologous Series

General Formula

Alkanes

CnH2n+2

 

Alkenes

CnH2n

Alcohols

CnH2n+1OH

Cycloalkanes

CnH2n

Cycloalkenes

CnH2n-2

Cycloalkanols

CnH2n-1OH

Aldehydes

CnH2nO

Ketones

CnH2nO

 

 Alkanes

 

·        Saturated hydrocarbons (Molecules made of carbon and hydrogen only and only contain single bonds)

 

·        Have a functional group C-C (A functional group is a group that reoccurs throughout a homologous series. In this case, alkanes are characterized by single C-C bonds, which are its functional group. A homologous series is a family of compounds that have the same functional group and similar chemical properties.)

 

·         Have the suffix –ane. The only difference between one alkane and the next is the addition of CH2 onto the chain, hence we get the formula CnH2n+2.

 

·        Are relatively unreactive(saturated). Can only participate in combustion or substitution reactions.

 

·        The first 4 are gases, then they become liquids (as more carbons are added, boiling points increase. This is because of the overall increase in Van der Waals forces, which are intermolecular forces.

 

Examples:

 

Methane                                  Ethane                                    Propane  

           

 

 

 

    CH4                                       C2H6                                         C3H8       

 

Notice how they follow the general formula.

 

Isomers

 

Isomers in alkanes start from butane, but here are the isomers of pentane:

 

 Pentane           

                                                                             2-Methylbutane

 

2,2-dimethylpropane

 

Isomers are molecules with the same molecular formula, yet different structural formula. They have similar chemical properties, but different physical properties.

 

The following steps are used in the naming process:

 

1.    Look for the longest chain of carbon atoms and write down its related alkane eg. 4 carbon atoms means butane

 

2.    Look for any side groups (CH3 groups called methyl groups) if there is one group, write methyl before the alkane in step 1, if two, write dimethyl, if three, trimethyl etc. eg. methylbutane

 

3.    Number the carbons from the end of the chain to the other end. You start numbering from the side with the closest methyl group. Now, look at what number that methyl group is on. If it is on the second carbon, place a number 2 before what you have written in step 4, and separate them with a hyphen. If you have more than one methyl group, just write the other numbers in, and separate them with commas. eg. 2,2-dimethylpropane. Notice how when we have two numbers, we have a “dimethyl”. This shows we have two methyl groups, and the numbers represent which carbon they are on (They are on the same one in this case)

 

Important Rules for finding isomers

 

1.    Never place a methyl group on carbon number one (carbons at the end of chains). No methyl groups can bond there as it is the end of the chain.

 

2.    Molecules can revolve around themselves, so a methyl group on top or the bottom of the second carbon is exactly the same thing.

 

Reactions of Alkanes

 

As we mentioned before, alkanes undergo substitution and combustion reactions only:

 

Combustion

 

All hydrocarbons burn. Alkanes are not very reactive, but they do burn to produce carbon dioxide and water.

 

eg. ethane + oxygen à carbon dioxide + water

         

2C2H6 + 7O2 à 4CO2 + 6H2O

 

Substitution

 

Alkanes can substitute their hydrogen atoms with halogens. When exposed to sunlight (UV light in particular) a mixture of methane and chlorine will react. The light splits up the chlorine molecules to form free radicals of chlorine (two separate ions) that readily displace a hydrogen from the methane:

 

CH4 + Cl2 à CH3Cl + HCl

 

This reaction can continue in excess chlorine to form dichloromethane, trichloromethane, tetrachloromethane.

 

Alkenes

 

·        Unsaturated hydrocarbons (Have double bonds therefore enabling them to participate in many different reactions).

 

·        Have a functional group of C=C

 

·        Have a suffix of –ene

 

·        They start from ethene, as a minimum of two carbons are needed to make a double bond.

 

·        The first 3 are gases, then they become liquids.

 

Examples:

 

Ethene                                              Propene                        But-1-ene

                                     

 

C2H4                                                                               C4H8

                                                        C3H6                               

 

Notice how they follow the general formula.

 

Isomers

 

They follow exactly the same rules as the Alkanes, but this time, it is depending on the position of the double bond within the molecule

 

Eg. Butene

 

But-1-ene                                         But-2-ene

                        

 

2-methylpropene

 

When the double bond is bonded to the first carbon (To the end of the chain) it is called but-1-ene. When it is bonded to the second, It is called but-2-ene.

 

 

 

 

 

Reactions of Alkenes

 

Alkenes participate in a wide range of reactions. They include:

 

Addition Reactions

 

In these reactions, atoms are added to the structure. The most known one is the one with bromine water, and I will refer to it later. Because it has double bonds, the alkene can break them to bond with other atoms.

 

C2H4 + Br2 à C2H4Br2

 

Notice how the bromine was just added to the formula.

 

Hydration

 

Commercially, alcohol is manufactured in this method. With the addition of water in form of steam (This is just another form of addition reaction) alcohol is formed.

 

C2H4 + H2O à C2H5OH

 

Alcohols

 

·        Are NOT hydrocarbons, as they have at least one oxygen atom in their molecule.

 

·        Have a hydroxyl functional group of –O-H

 

·        Have a general formula of CnH2n+1OH

 

What we need to know for topic 2 is more about the reactions of Alkenes. You should be able to find isomers easily if you’ve read the previous sections, but please read my document on ‘How to write molecular and structural formulae’ to see how to include the –O-H group in them.

 

 

Reactions of Alcohols

 

-Reactions with sodium

 

Sodium reacts with alcohols the same way as with water, but less vigorously. Reactivity reduces down the group. Because of this property, alcohol can be used to clean up sodium from a desk. When I say they react the same way as water, I can compare them in the following equations:

 

Na + H2O à NaOH + H2

 

Na + CH3COOH à CH3COO-Na+ + H2

 

An ionic substance is produced in the process, called an alkoxide. The reason why we put the alcohol group before the sodium is to make its origin obvious. We also place the signs to show its ionic. Ths product above is thus called Sodium ethoxide.

 

-Dehydration (elimination)

 

The reason why it’s so is because of the fact water is removed from it. It is also an elimination reaction.

 

What is important for this reaction is the apparatus, and equally important the catalyst used. The apparatus can be found on page 32 in the student book.

 

The usage of catalysts as well as apparatus usually depends on the product. If the product is a liquid, we use apparatus on page 39. If gas, we use the one on page 32. As for the catalysts, we use either aluminium oxide or porous pot for the gaseous products, and phosphoric acid or sulphuric acid for liquid products, but sulphuric tends to react, producing sulphates we would have to separate.

 

Ethanol à ethene + water