SYNTHESIS PART ONE HYDROCARBONS

What is organic chemistry? In chemistry chemicals which contain carbon are classed as organic. Carbon is a non metal and can form four bonds with other non metals. This means it can form many, many different substances in many different shapes. Some organic substances are simple like methane and others are very complex like sugars, proteins and DNA.

Hydrocarbons Organic compounds that contain only hydrogen and carbon are called hydrocarbons. There are two main groups you have to know about – alkanes and alkenes.

Alkanes The simplest alkane is called methane. It consists of one carbon atom joined to four hydrogen atoms We write its formula as CH 4 Carbon atom Hydrogen atom

The next alkanes in the series are Ethane C 2 H 6 Propane C 3 H 8 Butane C 4 H 10 As we add another carbon atom in the chain we need more hydrogen atoms. Can you see how these two are related? If we double the number of carbon atoms and add two we get the number of hydrogen atoms. CLICK HERE TO COMPLETE THE ALKANES WORKSHEET.CLICK HERE TO COMPLETE THE ALKANES WORKSHEET

The alkenes In these hydrocarbons one of the carbons has a double bond. This means that there are two bonds holding two of the carbons together instead of only one Double carbon carbon bond Carbon atom

Saturated & Unsaturated The alkanes have only single C-C bonds. We say they are saturated. The alkenes have a double C=C bond. We say they are unsaturated.

The simplest alkene is called ethene It has the formula C 2 H 4 There are always twice as many hydrogen atoms as carbon atoms in the alkenes. The other two alkanes you need to know are propene and butene CLICK HERE TO COMPLETE ALKENE WORKSHEET

We can test to see if a hydrocarbon is an alkene. Alkenes turn bromine water from orange/brown to colourless. H H I I I I C=C + Br 2 Br-C-C-Br I I H H Alkanes will not do this

CRACKING When crude oil is separated by fractional distillation there are always a higher % of the longer chained fractions. The shorter chained hydrocarbons are more useful so the long chained molecules are cracked to give more shorter chain hydrocarbons.

Cracking in the School Laboratory Paraffin is soaked into ceramic wool and placed at the end of a boiling tube (see diagram) The aluminium oxide is heated strongly first. This is the catalyst which speeds the reaction up. The paraffin is heated to vaporise it and then the ethene gas is collected in the test tubes. Click here to complete the cracking exercise Paraffin soaked into mineral wool CatalystEthene gas collecting heat

What can we do with the ethene? The ethene can be used as a fuel but because it contains a double bond but it is much more useful as a raw material in the chemical industry. If it is reacted with water(steam) ethanol (alcohol) can be made. H H H H I I I I C=C + H-O-H H-C-C-O-H I I I I H H H H ethene + steam ethanol

Polymers Ethene and other alkenes are most useful for making polymers. Plastics are polymers. Given the right conditions small alkenes can join together to form very large molecules. The small molecules are called monomers and the large molecules are called polymers. The process is called polymerisation Click here to complete the crossword

How are they made? Alkenes have a double C=C bond The double bond breaks and the smaller alkenes join together. Double bond breaks Alkenes join to form polymer ETHENE ETHENE ETHENEPOLYETHENE Click here to complete the polymer worksheet

There are two main types of polymer. They are thermosetting and thermoplastic polymers. Thermoplastic They can stretch or bend They soften & melt when heated Then can be remoulded into new shapes They have no cross links between the polymer molecules Thermosetting Are rigid, they break when bent They char or decompose when heated They cannot be moulded into new shapes They have cross links between the polymer molecules Click here to complete the matching exercise

Properties of polymers Polyethene & polypropene are made from unsaturated monomers. These monomers ADD on to each other so we call them addition polymers. Other polymers are formed by condensation. This means another molecule – water is made as well as the polymer. Some polymers have branched chains. This affects the strength of the polymer.

HDPE (high density polyethene) has few branches and is not as strong as LDPE (low density polyethene) which has many branches. Some times plasticisers have to be added to alter the properties. These allow the polymer chains to slide over each other which makes the polymer softer & more flexible. uPVC is the hard polymer used to make window frames. If plasticiser is added to this the polymer it produces a soft, flexible substance that can be used for floor covering.

Getting rid of polymers Some polymers are hard to dispose of. Thermoplastic polymers can be recycled Many polymers are not biodegradeable (will not rot) and are not affected by acids and other chemicals. There are however some new polymers around that have additives which allow them to decompose. Click to complete the cloze exercise