Reactions of Alkanes (pages 94-95) Flashcards
Alkanes react with particles that have unpaired electrons, what are these called?
free radicals.
These reactions happen in several steps and a mechanism breaks reactions down to show you the steps.
What is the name of ‘breaking a covalent bond’ called?
Bond Fission.
A single covalent bond is a shared pair of electrons between two atoms. What are the two ways it can break?
Heterolytic Fission
Homolytic Fission.
Explain what Herterolytic Fission is?
In herterolytic fission the bond breaks unevenly with one of the bonded atoms receiving both electrons from the bonded pair. Two different substances are formed - a positively charged cation (X+), and a negatively charged anion (Y-).
X ÷ Y 》》》 X+ + Y- (See diagram 1, a curly arrows show the movement of an electron pair)
(HETERO MEANS DIFFERENT)
Explain what Homolytic Fission is?
In homolytic fission, the bond breaks evenly and each bonding atom receives one electron from the bonded pair. Two electrically uncharged ‘radicals’ are formed. Radicals are particles that have an unpaired electron. They are shown in mechanism by a big dot next to the molecular formular (the dot represents the unpaired electron)
X÷Y 》》》 X• + Y•
Because of the unpaired electron, radicals are very reactive.
What are Photochemical reactions?
Photochemical reactions are started by light - this reaction requires ultraviolet to get going.
A hydrogen atom is substituted (replaced) by chlorine or bromine. This is a free-radical substitution reaction.
Example: chlorine and methane react with a bit of a bang to form chloromethane:
CH4 + CI2 uv»> CH3CI + HCI
Hologens react with Alkanes, forming what?
Haloalkanes
What are the three stages in a radical chain reaction
Initiation reactions - free radicals are produced
Propagation reactions - free radicals are used up and created in a chain reaction.
Termination reactions - free radicals are mopped up.
Explain how Free radicals are produced (initiation reactions) - first stage
1) sunlight provides enough energy to break the C1-C1 bond - this is photodissociation.
C2 uv»> 2C1• (see proper equation on page 94).
2) The bond splits equally and each atom gets to keep one electron - homolytic fission. The atom becomes a highly reactive free radical, C1• because of its unpaired electron.
Explain the Propagation reaction (free radicals are used up and created in a chain reaction) stage?
1) C1• attacks a methane molecule: Cl• + CH4 »_space;» •CH3 + HCl
2) The new methyl free radical: •CH3, can attack another Cl² molecule: •CH³ + Cl²»_space;> Ch³CI÷Cl•
3) The new Cl• can attack another CH⁴ molecule, and so on, until all the Cl² or Cl⁴ molecules are wiped out.
Explain the Termination reactions - (free radicals are mopped up) in the free radicals stage 3?
Termination reactions - free radicals are mopped up.
1) if two free radicals join together, they make a stable molecule.
2) There are heaps of possible termination reactions. Here are a couple of them to you you the idea:
Cl• + •CH3 »_space;> CH3Cl
•CH3 + •CH3 »_space;> C2H6 (some products formed will be trace impurities in the final sample)
The reaction between bromine and methane works in exactly the same way:
CH4 + Br2 Uv»»> CH3Br + HBr
What is the big problem with free-radical substitution if your’re trying to make a particular product?
you dont only get the product you’re after, but a mixture of products.
e.g. if you’re trying to make chloromethany and there’s too much chlorine in the reaction mixture, some of the remaining hydrogen atoms on the chloromethane molecule will be swapped for chlorine atoms. The propagation reactions happen again, but time to make ‘dichloromethane’
Cl• +CH3CL»_space;> •CH2Cl + HCl.
•CH2CL + Cl2 »_space;» CH2Cl2 + Cl• (dichloromethane)
It doesnt stop there. Another substituion reaction can take place to form trichloromethane:
Cl• CH2Cl2 »_space;» •CHCl2 + HCl
•CHCl2 + Cl2 »_space;» CHCl3 + Cl• (trichloromethane)
Tetrachloromethane (CCl4) is formed in the last possible substitution, what do this mean?
there are no more hydrogens attached to the carbon atom, so the substitution process has to stop.
soo the end product is a mixture of CH2Cl, CH2 Cl2, CHCL3 and CCl4..This is a nuisance, because you have to deparate the chloromethane from the other three unwanted by-products.
What is the best way of reducing the chance of by-products forming?
is to have an excess of methane. a methane molecule and not a chloromethane molecule.
What is another problem with free radical substitution, apart from by-products?
that is can take place at any point along the carbon chain. So a mixture of isomers can be formed. for example, reacting propane with chlorine will produce a mixture of 1-chloropropane and 2-chloropropane.
