alkanes

Alkanes

saturated hydrocarbons containing single covalent bonds, C-C and C-H bonds in alkane molecules as o-bonds

• General formula Cn H2n+2 

Sigma bonds

formed from the overlap of orbitals directly between the bonding atoms); free rotation of the o-bond, sigma bonds can freely rotate 

explanation of the tetrahedral shape and bond angle around each carbon atom in alkanes in terms of electron pair repulsion

In alkane molecules each carbon atom is surrounded by 4 bonded pairs of electrons, the 4 bonded pairs repel each other equally resulting in a tetrahedral shape around each carbon atom and bond angles at 109.5

explanation of the variations in BPs of alkanes with different carbon chain length in terms of induced dipole-dipole interactions (London forces)

As the carbon chain length increases BPs increase as there are more electrons and more points of contact between molecules

The strength of london forces between molecules increases

More energy is needed to break IMFs 

explanation of the variations in BPs of alkanes with different branching in terms of induced dipole-dipole interactions (London forces)

As branching in isomers of alkanes increases BPs decreases 

There are fewer points of contact between the molecules 

The strength of london forces between molecules decreases

Less energy is needed to break IMFs 

Solubility (not in spec)

alkanes are non polar molecules and don't dissolve in polar solvents like water, they will mix however with non polar solvents

the low reactivity of alkanes with many reagents in terms of the high bond enthalpy of the σ-bonds present

The C-C and C-H sigma bonds are very strong and hard to break, these bonds have a high bond enthalpy 

the low reactivity of alkanes with many reagents in terms of the very low polarity of the σ-bonds present

The C-C and C-H bonds are non polar, very low polarity as there is little difference in electronegativity between C and H atoms

complete combustion of alkanes, as used in fuels

Fuels are substances that can be burnt in oxygen to release heat energy, although alkanes are relatively unreactive many alkanes react exothermically with oxygen and make good fuels 

Complete combustion

in an unlimited supply of oxygen, alkanes burn completely to produce CO2 and water in an exothermic reaction - complete combustion

the incomplete combustion of alkane fuels in a limited supply of oxygen

in a limited supply of oxygen incomplete combustion occurs, the hydrogen in the hydrocarbon still forms water , the carbon only undergoes partial oxidation to form carbon monoxide, unburnt carbon particles can be released as carbon particles (soot) 

resulting potential dangers from CO

Carbon monoxide is a toxic gas that binds to haemoglobin in red blood cells preventing haemoglobin from carrying oxygen around the body, CO is difficult to detect as it is colourless and odourless 

the reaction of alkanes with chlorine and bromine by radical substitution using ultraviolet radiation

Methane reacts with chlorine in the presence of of UV radiation by radical substitution, overall reaction: CH4 + Cl2 —> CH3Cl + HCl 

including a mechanism involving homolytic fission and radical reactions in terms of initiation, propagation and termination

the radical substitution mechanism proceeds by 3 steps initiation, propagation, termination 

Initiation

UV provides energy for homolytic fission of chlorine molecules producing highly reactive chlorine radicals (Cl•) , in imitation step radicals are made 

Cl2 —> 2Cl •

Propagation

proceeds by two reactions producing the HCl and CH3Cl products, in propagation step radicals react with molecules to form new molecules and radicals  

Chlorine radicals react with methane to form HCl and methyl radicals (CH3•) 

Methyl radicals react with chlorine to form CH3Cl and 

Summary of propagation

Radicals are being consumed then being regenerated, propagation can carry on as a chain reaction, provided that there is a constant supply of reactants CH4 and Cl2 and a supply of UV, in the first reaction Cl• reacts and CH3• forms, if you add equations for the two propagation reactions you get overall equation for the reaction  

Termination

in each termination step two radicals react together to form a new molecule this happens naturally if two radicals collide and react , reaction then stops as there is no longer a constant supply of radicals, there are 3 possible termination reactions involving Cl• and CH3• radicals, 

3 equations

Cl• + Cl• —> Cl2 

•CH3 + Cl• —> CH3Cl 

•CH3 + •CH3 —> C2H6 

the limitations of radical substitution in organic synthesis by the formation of a mixture of organic products, in terms of further substitution and reactions at different positions in a carbon chains

Number 1- limitation in organic synthesis because a large mixture of the products can be formed 

limitation 2

Further substitution- the CH3Cl product still contains hydrogen atoms and these hydrogen atoms can be replaced by further substitution with chlorine to produce a mixture of other products 

Limitation 3

Formation of structural isomers: for alkanes with a chain of 3 or more carbon atoms substitution can take place at different positions along the carbon chain giving a mixture of structural isomers

EQ

Explain why the initiation step is an example of homolytic fission

The breaking of a Br-Br bond forms two radicals

The breaking of a Br-Br bond one electron from the bond pair goes to each atom

Explain the potential danger of incomplete combustion

Carbon monoxide gas is toxic

Determining whether the portable heater containing 600g of butane HC and consumes 5000 cm*3 of O2 at RTP is safe to use

Write out the equation for complete combustion of butane to determine how many moles needed of butane and oxygen for complete combustion to happen. Calculate moles needed of oxygen using molar gas vol and that and if the moles of oxygen needed is smaller than the moles of oxygen present: incomplete combustion will take place, it is not safe to use