Chapter 12
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What type of covalent bond are between the atoms in alkanes?
• Sigma bonds (σ-bonds)
What are σ-bonds?
• The result of a head-on overlap of 2 orbitals (either s or p), directly between the bonding atoms • (Each orbital contains 1 electron, so the σ-bond contains 2 shared electrons)
How do the boiling points of alkanes change?
• Their boiling points increase as the chain length increases
Why do the boiling points of alkanes increase as their chain length increases?
• Larger molecules have a larger surface area, and therefore form more contact points with other molecules • As a result, there is more surface contact between longer chain alkanes • The London forces between them are therefore stronger (as there are more of them), and more energy is required to overcome them • They then have a higher boiling point
How and why does branching affect the boiling point of alkanes?
• It decreases the boiling point • This is because there is a smaller surface area of contact between branched molecules (they cannot be packed as closely together as straight chain alkanes) • There are therefore fewer London forces • Less energy is needed to overcome them • They have a lower boiling point
Why are alkanes very unreactive?
• C-C and C-H σ-bonds have high bond enthalpy (are strong), and so are difficult to break • C-C bonds are non-polar (they don’t have a dipole, so are less likely to react) • C and H are similar enough to each other in terms of electronegativity that C-H bonds are also non-polar
What are the 2 types of combustion, and when does each occur?
• Complete combustion: when there is a plentiful supply of oxygen • Incomplete combustion: in a limited supply of oxygen
List the reactants and products in the complete combustion of alkanes.
• Alkane and oxygen • Carbon dioxide and water
List the reactants and products in the incomplete combustion of alkanes, and explain why they are formed.
• Alkane and oxygen • Water is always produced as the hydrogen atoms are always oxidised • The combustion of the carbon is incomplete, so either carbon monoxide or carbon (as soot) are produced
Why is CO dangerous?
• Carbon monoxide is colourless, odourless and highly toxic
When do alkanes react with halogens, and why?
• In the presence of UV radiation, as it provides the initial energy to start the reaction
What is the reaction of a halogen with an alkane an example of?
• Radical substitution
How many steps are there in radical substitution, and what are their names?
• 3 • Initiation • Propagation • Termination
In the example of methane reacting with bromine, what happens in the initiation stage? What is this process an example of?
• Br-Br -UV-> ·Br + ·Br • (Homolytic fission)
What happens in the propagation stage of methane’s reaction with bromine?
• ·Br + CH4 -> ·CH3 + HBr • ·CH3 + Br2 -> CH3Br + ·Br
What happens in the termination stage?
• 2 radicals collide, which forms a molecule with all electrons paired There are multiple possible termination steps: • ·Br + ·Br -> Br2 • ·CH3 + ·CH3 -> C2H6 • ·CH3 + ·Br -> CH3Br
What is a chain reaction, and how does this relate to the reaction between halogens and alkanes?
• A reaction in which the propagation steps release new radicals that continue the reaction • This is true for the reaction between halogens and alkanes
What are the limitations of radical substitution in organic synthesis?
• Further substitution can take place • Substitution can happen at different points in the carbon chain • If either of these things happen, a mixture of organic products is formed
How can further substitution take place?
• Br2 can collide with CH3Br again, which would then create CH2Br2
Give an example of the issue caused by substitution at different points in the carbon chain.
• For example, you can have 1-bromopentane, 2-bromopentane and 3-bromopentane