STPM SEM 3 CHEMISTRY NOTE - CHAPTER ALKANES

HYDROCARBONS 15.1 ALKANES

Alkane Isomers  The first three members of alkanes (CH4 = methane, C2H6 =

ethane, C3H8= propane) have only one structural formula each and do not exhibit structural isomerism.  The molecular formula of an alkane with more than three carbons can give more than one structure  C4 (butane) = butane and isobutane  C5 (pentane) = pentane, 2-methylbutane, and 2,2dimethylpropane  Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes  Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes 2

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 For example, C4H10 exhibits 2 chain isomers and C6H14: .

 What about C5H12? How many isomers and give the name for each

one?

Isomerism in cycloalkanes  Cycloalkanes again only contain carbon-hydrogen bonds and carbon-

carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane.  If you count the carbons and hydrogens,

you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose two hydrogen atoms.You are unlikely to ever need it, but the general formula for a cycloalkane is CnH2n.

Cis and trans isomers

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may exist in two types of molecules: 1) Cycloalkanes with substituents 2) Alkenes Rotation around the bonds in a cyclic structure is limited. The formation of cis-trans isomers or geometric isomers, is a consequence of the absence of the free rotation.

 Cycloalkanes are less flexible than open-chain alkanes

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 Much less conformational freedom in cycloalkanes

Combustion & Halogenation of alkanes  Alkanes are fuels; they burn in air if ignited. Complete

combustion gives carbon dioxide and water; less complete combustion gives carbon monoxide or other less oxidized forms of carbon.  Alkanes react with halogens(chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free radical chain mechanism.

Complete combustion  Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.  In order to make the equations, for example, with propane (C3H8), we can balance the carbons and hydrogens as we write the equation down:

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 Counting the oxygens leads directly to the final version:

 The hydrocarbons become harder to ignite as the molecules get bigger.

This is because the bigger molecules don't vaporise so easily - the reaction is much better if the oxygen and the hydrocarbon are well mixed as gases. If the liquid isn't very volatile, only those molecules on the surface can react with the oxygen.

 Bigger molecules have greater Van der Waals attractions which makes it

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more difficult for them to break away from their neighbours and turn to a gas.  Provided the combustion is complete, all the hydrocarbons will burn with a blue flame. However, combustion tends to be less complete as the number of carbon atoms in the molecules rises. That means that the bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame. Incomplete combustion  Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.  As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over!  The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colourless poisonous gas.

Why carbon monoxide is poisonous?

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 Oxygen is carried around the blood by haemoglobin (US:

hemoglobin). Unfortunately carbon monoxide binds to exactly the same site on the haemoglobin that oxygen does.  The difference is that carbon monoxide binds irreversibly making that particular molecule of haemoglobin useless for carrying oxygen. If you breath in enough carbon monoxide you will die from a sort of internal suffocation.

Free radical substitution- halogenation .

The reaction between alkanes and fluorine  This reaction is explosive even in the cold and dark, and you tend to get carbon and hydrogen fluoride produced. It is of no particular interest. For example:

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The reaction between alkanes and iodine  Iodine doesn't react with the alkanes to any extent - at least, under normal lab conditions. The reactions between alkanes and chlorine or bromine  There is no reaction in the dark.  In the presence of a flame, the reactions are rather like the fluorine one producing a mixture of carbon and the hydrogen halide. The violence of the reaction drops considerably as you go from fluorine to chlorine to bromine.  The interesting reactions happen in the presence of ultra-violet light (sunlight will do). These are free radical reaction, and happen at room temperature.

A Free Radical Substitution Reaction  If a mixture of methane and chlorine is exposed to a flame, it

explodes - producing carbon and hydrogen chloride. This isn't a very useful reaction!  The reaction we are going to explore is a more gentle one between methane and chlorine in the presence of ultraviolet light - typically sunlight. This is a good example of a photochemical reaction - a reaction brought about by light. CH4 + Cl2 CH3Cl + HCl  The organic product is chloromethane. One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms.

Cycloalkanes  The. reactions of the cycloalkanes are generally just the same as the alkanes, with the exception of the very small ones - particularly cyclopropane. The extra reactivity of cyclopropane  In the presence of UV light, cyclopropane will undergo substitution reactions with chlorine or bromine just like a non-cyclic alkane. However, it also has the ability to react in the dark.  In the absence of UV light, cyclopropane can undergo addition reactions in which the ring is broken. For example, with bromine, cyclopropane gives 1,3-dibromopropane.

 This can still happen in the presence of light - but you will get

substitution reactions as well. .  The ring is broken because cyclopropane suffers badly from ring strain. The bond angles in the ring are 60° rather than the normal value of about 109.5° when the carbon makes four single bonds.  The overlap between the atomic orbitals in forming the carboncarbon bonds is less good than it is normally, and there is considerable repulsion between the bonding pairs. The system becomes more stable if the ring is broken.

Catalytic Converters  A catalytic converter is a device used to reduce the harmful emissions 

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from an internal combustion engine A catalyst is a substance that changes the rate of a chemical reaction, but whose own composition is unchanged by that reaction. For air pollution control purposes, such reactions involve the reduction of nitric oxide to molecular oxygen and nitrogen or oxidation of hydrocarbons and carbon monoxide to carbon dioxide and water. Using the catalyst, the activation energy of the desired chemical reaction is lowered. Therefore, exothermic chemical conversion will be favored at a lower temperature. Traditional catalysts have normally been metallic, although nonmetallic materials, such as ceramics, have been coming into use in recent years.

 Metals used as catalysts may include noble metals, such as platinum, or

base metals, including nickel and copper.  Some catalysts are more effective in oxidation, others are more effective in reduction.  Some metals are effective in both kinds of reactions.  The catalyst material is normally coated on a porous, inert support structure of varying design.

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Hydrocarbons help contribute to smog. Smog is nothing more than a mixture of fog, smoke, and chemical fumes which is definitely bad for the environment and it's not healthy for us to breathe. Carbon monoxide which is an odorless, colorless gas but it is very toxic. Carbon monoxide formulates when there is an incomplete oxidation of carbon during the combustion process. This gas can lead to fatigue and even chest pains and when the dosage increases your vision becomes impaired along with headaches, nausea, confusion, and dizziness. And of course at the most intense level of concentration someone can die. Through the combustion of gasoline carbon monoxide is created. The next pollutant is nitrogen oxide which is created when the extreme heat from a car engine forces nitrogen to mix with oxygen resulting in smog and acid rain.

A catalytic converter is a very simple device using the basic redox reactions in chemistry to help reduce the pollutants a car makes. It converts around 98% of the harmful fumes produced by a car engine into less harmful gases.

Functions A three-way catalytic converter has three simultaneous functions: Reduction of nitrogen oxides into elemental nitrogen and oxygen. NOx → Nx + Ox Oxidation of carbon monoxide to carbon dioxide. CO + O2 → CO2 Oxidation of hydrocarbons into carbon dioxide and water. CxH4x + 2xO2 → xCO2 + 2xH2O

Requirements of Catalytic Converter  The goal is to channel exhaust over a large surface area of 

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catalyst without an unacceptable pressure drop. The reduction and oxidation processes can be conducted sequentially or simultaneously. Dual catalysts are used in sequential reduction-oxidation. In this case, the exhaust gas from a rich-burn engine initially enters the reducing catalyst to reduce nitric oxide. Subsequently, as the exhaust enters an oxidation catalyst, it is diluted with air to provide oxygen for oxidation. Alternatively, three-way catalysts can be used for simultaneous reduction and oxidation. Engines exhausting to such catalysts run slightly rich and require tight regulation of air-fuel ratio.