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#alkanes

This week has been tough. I haven’t posted anything in a while because at the end of last week my school was hit with some really personal information and that last Thursday and Friday, everybody in school was emotionally exhausted, so it’s been tough. On Friday I did however leave school to attend the climate strike in my town and then I went to my boyfriend’s house because it was Valentines day. But to say I wasn’t hopeful at the start of this week to get loads of work done, would be a lie. I planned on doing a past paper for each of my subjects everyday - we were off for the week because it is mid-term - however, as per usual, I set up this workload thinking I’d actually be able to do it. I would love to say that I worked my ass off everyday this week, bu that would be the biggest load of bullshit ever; I didn’t properly start working till yesterday. I am proud of my chemistry notes I made yesterday, however, next week is our data tracking 3 (dt3) and on Monday I have biology…the one subject I’ve basically done nothing for. On Monday I have biology at 1:00, I have Tuesday off, Wednesday I have chemistry at 9:15, Thursday I have maths at 9:15 and finally on Friday I have physics at 9:15. I’m fucked. I don’t really know how to answer the questions from AS2 in biology and that whats most of the paper is going to be on. I also have physics homework but that won’t be due to till next week. On Wednesday afternoon my year has to go to this weird careers talk but I no longer have to go, but I already said I could, so next week is really not going to be a vibe. So I’m probably not going to post anything till next Saturday but I think that’s fair due to my exam timetable. Also not to flex but I’m proud of my chemistry mind-maps:

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Haloalkanes are more commonly referred to as halogenoalkanes. Obviously you’ve already read my post on halogenoalkanes and their properties so there’s no surprise that you’re itching to read what I’ve got to say about these beauties and their reactions! Should we delve in?

There are a few different kinds of reactions you must learn for the A Level exam that involve halogenoalkanes. 

The first is the synthesis of chloroalkanes via the photochemical chlorination of the alkanes. I know it looks scary, but don’t worry, it is simpler than it sounds. It essentially means “forming chloroalkanes through chlorinating an alkane in the presence of sunlight”.

Chlorine will react with methane when UV light is present and will form several kinds of chloroalkanes and fumes of hydrogen chloride gas. Chloromethane was once commonly used as a refridgerant. Depending on how many chlorine molecules there are, there will be different compounds formed:

methane + chlorine -> chloromethane + hydrogen chloride

CH4 + Cl2 -> CH3Cl + HCl

or

methane + chlorine -> trichloromethane + hydrogen chloride

CH4 + 3Cl2 -> CHCl3 + 3HCl

When undergone in real life, mixtures of halogenoalkanes are produced with some long chain alkanes which can be separated out with fractional distillation

To understand what happens in an overall chemical reaction, chemists use mechanisms. These basically show the step-by-step process that is usually shown by a simple symbol equation that summarises everything.

The chlorination of methane is something you must learn the mechanism for. It’s pretty easy but involves a lot of steps and must be revised periodically to remember them.

The actual reaction is a substitution reaction because one atom or group is replaced by another. Since the chlorine involved is a free radical, it can also be called a free-radical substitution reaction.

1. Initiation

UV light is essential for the first step in the mechanism. This breaks the Cl-Cl covalent bond so that each chlorine leaves with one electron from the shared pair. Chlorine free radicals, with one unpaired electron in the outer shell, are formed. Free radicals are only formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.

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2. Propagation

This has two sub-steps

(a) Chlorine free radicals (highly reactive) react with methane to form hydrogen chloride and leave a methyl free radical.

Cl• + CH4 -> HCl + •CH3

(b) This free radical then reacts with another chlorine to form chloromethane and another chlorine free radical. Producing free radicals is a chain reaction which is why it is such a problem in ozone depletion - a little amount can cause a lot of destruction.

•CH3 + Cl2 -> CH3Cl +  •Cl

3. Termination

This step stops the chain reaction. It only happens when two free radicals collide to form a molecule in several ways:

Cl• + Cl• -> Cl2

UV light would just break down the chlorine molecule again, so although this is technically a termination reaction it is not the most efficient.

Cl• +  •CH3 -> CH3Cl

Forming one molecule of methane uses one chlorine and one methyl free radical.

•CH3 +  •CH3 -> C2H6

Ethane can be formed from two methyl free radicals - this is why there are longer chain alkanes in the mixture. 

This whole process is how organic halogenoalkanes are the product of photochemical reactions of halogens with alkanes in UV light - made via free radical substitution mechanisms in chain reaction.

Another reaction you need to know is a nucleophilic substitution reactions. A nucleophile is an electron pair donor or proton acceptor - the name comes from Greek origins (”loves nucleus”) - such as hydroxide ions, cyanide ions or ammonia molecules. Hydroxide and cyanide ions are negative but ammonia is neutral.

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Halogenoalkanes have a polar bond because of the difference between the highly electronegative halogen and the carbon atom. The 𝛿+ carbon can go under nucleophilic attack. The mechanism for negatively charged nucleophiles these in general is:

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Nu represents the nucleophile. This example is with a bromoalkane. Make sure to include curly arrows that begin at a lone pair or the centre of a bond and end at an atom or centre of bond, and delta (slight) charges.

Lets look at a more specific example:

One nucleophile that can be used is a hydroxide ion, found in either water or sodium hydroxide. In this case, you need to know about aqueous sodium hydroxide or potassium hydroxide and a halogenoalkane. This takes place at room temperature but is slow so is often refluxed (continuously boiled and condensed back into the reaction flask). Reflux apparatus is shown below:

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The halogenoalkane is dissolved into ethanol since it is insoluable in water and this solution along with the aqueous hydroxide can mix. The product produced is an alcohol, which is organic.

The general reaction is:

R-CH2X + NaOH -> CH3CH2OH + NaX

Where X represents a halogen.

You must learn the mechanism for this reaction. The lone pair on the hydroxide attacks the carbon atom attached to the halogen and this causes both carbon electrons to move to the halogen which becomes a halide ion.

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The reaction of a hydroxide ion can also be classed as a hydrolysis reaction as it breaks down chemical bonds with water or hydroxide ions. The speed of reaction depends on the strength of the bond - a stronger carbon-halogen bond, a slower reaction.

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C-I is the most reactive (reactivity increases down group 7) and C-F is therefore the least reactive and strongest.

Part two of this post will cover nucleophilic substitution of cyanide ions and ammonia molecules, as well as elimination reactions.

SUMMARY

  • You need to know about the synthesis of chloroalkanes via the photochemical chlorination of the alkanes. - “forming chloroalkanes through chlorinating an alkane in the presence of sunlight”.
  • Chlorine will react with methane when UV light is present and will form several kinds of chloroalkanes and fumes of hydrogen chloride gas. Depending on how many chlorine molecules there are, there will be different compounds formed.
  • When undergone in real life, mixtures of halogenoalkanes are produced with some long chain alkanes which can be separated out with fractional distillation
  • To understand what happens in an overall chemical reaction, chemists use mechanisms. These basically show the step-by-step process.
  • The chlorination of methane is something you must learn the mechanism for. The actual reaction is a substitution reaction because one atom or group is replaced by another. 
  • The first step is initiation UV light is essential for the first step in the mechanism. This breaks the Cl-Cl covalent bond so that each chlorine leaves with one electron from the shared pair. Chlorine free radicals, with one unpaired electron in the outer shell, are formed. Free radicals are only formed if a bond splits evenly - each atom getting one of the two electrons.
  • Step two is propagation: (a) Chlorine free radicals (highly reactive) react with methane to form hydrogen chloride and leave a methyl free radical (b) this free radical then reacts with another chlorine to form chloromethane and another chlorine free radical. Producing free radicals is a chain reaction which is why it is such a problem in ozone depletion - a little amount can cause a lot of destruction.
  • To stop the chain reaction, the final step is termination. It only happens when two free radicals collide to form a molecule in several ways: two chlorine free radicals forming a chlorine molecule, two methyl FRs forming ethane or a chlorine FR and a methyl FR forming chloromethane.
  • Ethane contributes to the longer chain alkanes in the mixture. 
  • Another reaction you need to know is a nucleophilic substitution reactions. A nucleophile is an electron pair donor or proton acceptor, such as hydroxide ions, cyanide ions or ammonia molecules. Hydroxide and cyanide ions are negative but ammonia is neutral.
  • Halogenoalkanes have a polar bond because of the difference between the highly electronegative halogen and the carbon atom. The 𝛿+ carbon can go under nucleophilic attack
  • Nu represents the nucleophile. Make sure to include curly arrows that begin at a lone pair or the centre of a bond and end at an atom or centre of bond, and delta (slight) charges.
  • One nucleophile that can be used is a hydroxide ion, found in either water or sodium hydroxide. In this case, you need to know about aqueous sodium hydroxide or potassium hydroxide and a halogenoalkane. This takes place at room temperature but is slow so is often refluxed (continuously boiled and condensed back into the reaction flask). The halogenoalkane is dissolved into ethanol since it is insoluable in water and this solution along with the aqueous hydroxide can mix. The product produced is an alcohol, which is organic.
  • The general reaction is :R-CH2X + NaOH -> CH3CH2OH + NaX where X represents a halogen
  • The lone pair on the hydroxide attacks the carbon atom attached to the halogen and this causes both carbon electrons to move to the halogen which becomes a halide ion.
  • The reaction of a hydroxide ion can also be classed as a hydrolysis reaction as it breaks down chemical bonds with water or hydroxide ions. 
  • The speed of reaction depends on the strength of the bond - a stronger carbon-halogen bond, a slower reaction. C-I is the most reactive (reactivity increases down group 7) and C-F is therefore the least reactive and strongest.
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So you want to be an organic chemist? Well, learning about hydrocarbons such as alkanes is a good place to start…

Alkanes are a homologous series of hydrocarbons, meaning that each of the series differs by -CH2 and that the compounds contain carbon and hydrogen atoms only. Carbon atoms in alkanes have four bonds which is the maximum a carbon atom can have - this is why the molecule is described to be saturated. Saturated hydrocarbons have only single bonds between the carbon atoms.

The general formula of an alkane is CnH2n+2 where n is the number of carbons. For example, if n = 3, the hydrocarbon formula would be C3H8 or propane. Naming alkanes comes from the number of carbons in the chain structure.

Here are the first three alkanes. Each one differs by -CH2.

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Shorter chain alkanes are gases at room temperature, medium ones are liquids and the longer chain alkanes are waxy solids.

Alkanes have these physical properties:

1. They are non-polar due to the tiny difference in electronegativity between the carbon and hydrogen atoms.

2. Only Van der Waals intermolecular forces exist between alkane molecules. The strength of these increase as relative molecular mass increases therefore so does the melting/boiling point.

3. Branched chain alkanes have lower melting and boiling points than straight chain isomers with the same number of carbons. Since atoms are further apart due to a smaller surface area in contact with each other, the strength of the VDWs is decreased.

4. Alkanes are insoluble in water but can dissolve in non-polar liquids like hexane and cyclopentane. Mixtures are separated by fractional distillation or a separating funnel.

The fractional distillation of crude oil, cracking and the combustion equations of the alkanes will be in the next post.

SUMMARY

  • Alkanes are a homologous series of hydrocarbons. Carbon atoms in alkanes have four bonds which is the maximum a carbon atom can have - this is why the molecule is described to be saturated. Saturated hydrocarbons have only single bonds between the carbon atoms.
  • The general formula of an alkane is CnH2n+2 where n is the number of carbons.
  • Shorter chain alkanes are gases at room temperature, medium ones are liquids and the longer chain alkanes are waxy solids.
  • They are non-polar.
  • Only Van der Waals intermolecular forces exist between alkane molecules. The strength of these increase as relative molecular mass increases therefore so does the melting/boiling point.
  • Branched chain alkanes have lower melting and boiling points than straight chain isomers with the same number of carbons.
  • Alkanes are insoluble in water but can dissolve in non-polar liquids like hexane. Mixtures are separated by fractional distillation or a separating funnel.
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Cyclohexane & hexane

Burning

  • hexane: clean, large bright orange flame, combusts easily
  • cyclohexane: slightly dirty, large bright orange flame (larger than hexane), combusts easily. Burns longer than hexane

Reacting with Bromine Water

  • both cyclohexane and hexane have no visible reaction
  • immiscible layer

 Reacting with Potassium Manganate (VII)

  • both cyclohexane and hexane have no visible reaction
  • immiscible layer
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Homologous Series

Organic compounds can be placed into various well-defined groups called homologous series e.g alkanes, alkenes and alcohols. The members of a homologous series conform to the following:

. They all have the same general formula, that is, a common relationship exists between the numbers of different atoms in these compounds.

. Each member differs from its nearest neighbour by a molecular mass of 14 or a CH2 group.

. They all have the same functional group. For example, all alkenes have a double bond whereas all the alcohols have a hydroxyl group(OH)

. They have similar chemical properties because reactivity is determined by the functional group and since they have the same functional group they will react similarly.

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The test for unsaturation is:
-Add Bromine water to the alkane/alkene and shake
RESULT
ALKANES (Saturated)- Stays as brown solution
ALKENES (Unsaturated)- Colour disappears (decolourizes)

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