Organic Chemistry for A-Level: Hydrocarbons, Haloalkanes and Alcohols
In this section, we will try to see the reaction in organic chemistry and also the mechanisms. You will notice in later part there will be a lot of arrows in the mechanisms. Those arrows have meanings, so please pay attention about the arrows below.
In incomplete combustion, the reaction produces carbon monoxide. This reaction mainly happen in a closed vessel which causes oxygen for combustion is limited.
Second reaction is complete combustion, which is the reaction is shown below.
In this reaction, alkane reacts with excess oxygen. Excess of oxygen could be provided by using open vessel or by reacting alkane with high pressure of oxygen. The complete combustion produces carbon dioxide which is less harmful for human instead carbon monoxide.
Free radical could be defined as an atom or group of atoms with an unpaired electron. The free radical substitution has 3 main steps; initiation, propagation, and termination.
In initiation step, the electrons in Cl-Cl bond are divided equally, so it is called homolytic fission. The Cl radicals react with with methane in this case to form methyl radical. This step a radical is formed from a radical atom so this step is called propagation. In the termination the desired product CH3Cl is formed from 2 radicals to form an inactive product; hence the reaction is terminated. However, since the nature of unpaired electron in radical reaction is very reactive, side products could also be formed.
If the reaction involves alkyl halide it is called dehydrohalogenation reaction.
Both of the reactions are called elimination reaction. The other way to prepare alkenes is from cracking reaction.
i. Addition with hydrogen
Ni catalyst is required since the bonding in H2 is very strong.
ii. Reaction with halogens
Example:
The reaction happens via:
As bromine molecule approaches an alkene, the electron density of the alkene pi bond repels electron density in the closer bromine, polarising the bromine molecule and making the closer bromine atom electrophilic. The alkene donates a pair of electrons to the closer bromine, causing displacement of the distant bromine atom. As this occurs, the newly bonded bromine atom, due to its size and polarisability, donates an electron pair to the carbon to form 3-membered ring bromonium ion intermediate.
Second step is a bromide anion attacks at the back side of one carbon (or the other) of the bromonium ion in an SN2 reaction, causing the ring to open and resulting in the formation of dibromoalkane.
iii. Reaction with hydrogen halides.
The π electrons of the alkene form a bond with a proton from HX to form a carbocation and halide ion. The halide ion reacts with the carbocation by donating an electron pair; the result is an alkyl halide. If the alkene is not symmetrical as shown in the example, more than one possible product could be formed. Therefore, the problem is which one is the major product.
The major product could be predicted from its intermediate. More stable intermediate would lead to the major product. More stable carbocation is caused by the positive charge is stabilised by the carbon chain next to the carbocation. More carbon chain next to the carbocation, more stable the carbocation is. Therefore, the series of carbocation stability is:
This series can be used in nuclephilic substitution of alkyl halide.
iv. The reaction with steam (water vapour) with acid catalyst
The alkene donates an electron pair to a proton to form the more tertiary carbocation. The carbocation reacts with a molecule of water to form a protonated alcohol. A transfer of a proton to a molecule of water leads to the product.
i. Oxidation with cold basic permanganate(VII) ions would lead to formation of a diol.
ii. Oxidation with hot basic permanganate(VII) ions would lead to a cleavage of C=C bond to form carboxylic acids.
Because alkynes has relatively high electron density, so the hydrogen atom that attaches to carbon alkynes would have relatively acidic properties. However, it is required a very strong base to show its acid properties.
Since alkyne has π bond, so it would react in almost similar way with alkenes. For example, alkynes could react with hydrogen in addition reaction to form alkene.
Where X is halogen atom (F, Cl, Br, or I)
i. SN2 mechanism
For SN2 reaction mainly happen in the primary alkyl halide. The reaction is called bimolecular because the rate of the reaction depends on both alkyl halide and nucleophile. In the transition state, the leaving group is still attached but it also starts to brake and the nucleophile is starting to form the covalent bond with the carbon atom. This process of moving out of the leaving group and creating the bond of nucleophile are in the same moment. Therefore the mechanism is:
From the mechanism above, this mechanism does not have an intermediate as the cleavage of C-Br bond and the formation of C-OH happens at the same time which is called concerted reaction. The consequences of this mechanism are the nucleophile will react faster with less hindered alkyl halide and if the starting material is enantiomeric pure, the product will have the inverse configuration of the starting material.
The relative rate of SN2 reaction can be summarised as:
ii. SN1 mechanism
It is called SN1 reaction because of the rate of reaction in this mechanism only depends on the alkyl halide, concentration of nucleophile has no effect. In this mechanism, the leaving group moves out first and then followed by formation of covalent bond by nucleophile. This is caused by the geometrical arrangement is sterically hindered, so there is not enough room for nucleophile to attack. Therefore, the mechanism is:
The formation of carbocation has several consequences. Firstly, it would react faster with more substituted carbocation (i.e. tertiary carbocation). Secondly, if the starting material is enantiomeric pure, the product would roughly a racemic mixture due to lost of stereocentre in intermediater. In addition, the series of relative rate in SN1 reaction is:
To summarise the substitution reaction:
ii. Elimination
Primary alcohols
Secondary alcohols
Tertiary alcohols
Alcohols could be formed mainly in three ways. Firstly, with substitution reaction of alkyl halide with OH- or with water.
Besides that, alcohols could also be synthesised from reaction of alkene with steam and acid catalyst. Lastly, in the main industry of alcohols, it could be made from fermentation reaction.
This reaction also could be used for identification of alcohol’s classification. The reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid; however it can be used for low molecular weight only. This reagent is called Lucas’ reagent and the observation is the relative rate. For example:
The mechanism will be discussed later in the section about carboxylic acid derivates.
This reaction has a function to identify CH3CH(OH)– group.
2) Reaction with sodium metal
Alcohols also have acid properties, but it is very weak acid (pKa ethanol = 15.1). Therefore, alcohol still can react with Na metal but not with NaOH.
3) Oxidation reaction (Reaction with acidified K2Cr2O7)
- The direction of the arrows is important.
- The shape of the arrow is important as well.
A. Aliphatic Hydrocarbons
Aliphatic hydrocarbons include alkanes, alkenes, and alkynes. Those organic compounds only consists carbon and hydrogen chain, and sometimes it could form cyclic structures as well. However, if the cyclic has resonance structure, such as benzene, it is called aromatic hydrocarbon (which will discuss later).1. Alkanes
Basically, most of the time alkanes are used for fuel. Therefore, the first reaction is combustion.a. Combustion reaction
Mainly, there are two types of combustion reaction of alkanes. Firstly is incomplete combustion and the reaction is shown below.In incomplete combustion, the reaction produces carbon monoxide. This reaction mainly happen in a closed vessel which causes oxygen for combustion is limited.
Second reaction is complete combustion, which is the reaction is shown below.
In this reaction, alkane reacts with excess oxygen. Excess of oxygen could be provided by using open vessel or by reacting alkane with high pressure of oxygen. The complete combustion produces carbon dioxide which is less harmful for human instead carbon monoxide.
b. Cracking reaction
This reaction happens in refinery of crude oil. The long chain of hydrocarbon is heated and with help of catalyst to brake the bond and forms shorter chain of hydrocarbon. For example is shown in the reaction below.
c. Free radical substitution
This reaction mainly has an objective to substitute hydrogen atom in alkanes with mainly is halogens (group 17). To substitute the hydrogen atoms with halogen atoms, it requires photon energy to trigger the reaction. Photon energy is needed to brake the bond because the between carbon and hydrogen atoms has slightly different in electronegativity, so the bond nearly has 0 moment dipole. Therefore the bond is slightly non-polar and it makes the bond is relatively strong. The breaking of this bond using photons produces free radical atoms.Free radical could be defined as an atom or group of atoms with an unpaired electron. The free radical substitution has 3 main steps; initiation, propagation, and termination.
In initiation step, the electrons in Cl-Cl bond are divided equally, so it is called homolytic fission. The Cl radicals react with with methane in this case to form methyl radical. This step a radical is formed from a radical atom so this step is called propagation. In the termination the desired product CH3Cl is formed from 2 radicals to form an inactive product; hence the reaction is terminated. However, since the nature of unpaired electron in radical reaction is very reactive, side products could also be formed.
2. Alkenes
Alkenes could be prepared from another organic compound such as alcohols or alkyl halides. The reaction involves alcohols to form an alkene is called dehydration of alcohols and it is shown below.
If the reaction involves alkyl halide it is called dehydrohalogenation reaction.
Both of the reactions are called elimination reaction. The other way to prepare alkenes is from cracking reaction.
a. Addition reaction
Alkenes could react to form the single-bonded organic compound. The reaction is called addition reaction, since the atoms are added to double bond to form single bond. The reagents in this reaction attacks the highly density of electrons (the double bond). Since the reaction attacks the highly density of electrons, so it is required at least partial positive charge in the molecule which is called electrophile. Therefore, it is called electrophilic addition.i. Addition with hydrogen
Ni catalyst is required since the bonding in H2 is very strong.
ii. Reaction with halogens
Example:
The reaction happens via:
As bromine molecule approaches an alkene, the electron density of the alkene pi bond repels electron density in the closer bromine, polarising the bromine molecule and making the closer bromine atom electrophilic. The alkene donates a pair of electrons to the closer bromine, causing displacement of the distant bromine atom. As this occurs, the newly bonded bromine atom, due to its size and polarisability, donates an electron pair to the carbon to form 3-membered ring bromonium ion intermediate.
Second step is a bromide anion attacks at the back side of one carbon (or the other) of the bromonium ion in an SN2 reaction, causing the ring to open and resulting in the formation of dibromoalkane.
iii. Reaction with hydrogen halides.
The π electrons of the alkene form a bond with a proton from HX to form a carbocation and halide ion. The halide ion reacts with the carbocation by donating an electron pair; the result is an alkyl halide. If the alkene is not symmetrical as shown in the example, more than one possible product could be formed. Therefore, the problem is which one is the major product.
The major product could be predicted from its intermediate. More stable intermediate would lead to the major product. More stable carbocation is caused by the positive charge is stabilised by the carbon chain next to the carbocation. More carbon chain next to the carbocation, more stable the carbocation is. Therefore, the series of carbocation stability is:
This series can be used in nuclephilic substitution of alkyl halide.
iv. The reaction with steam (water vapour) with acid catalyst
The alkene donates an electron pair to a proton to form the more tertiary carbocation. The carbocation reacts with a molecule of water to form a protonated alcohol. A transfer of a proton to a molecule of water leads to the product.
b. Reactions with basic permanganate(VII) ions
The condition would determine the product.i. Oxidation with cold basic permanganate(VII) ions would lead to formation of a diol.
ii. Oxidation with hot basic permanganate(VII) ions would lead to a cleavage of C=C bond to form carboxylic acids.
c. Polymerisation
The alkenes could form a polymer and it is called addition polymer.
3. Alkynes
The alkynes could be prepared by dehydrohalogenation of dialkyl halide.Because alkynes has relatively high electron density, so the hydrogen atom that attaches to carbon alkynes would have relatively acidic properties. However, it is required a very strong base to show its acid properties.
Since alkyne has π bond, so it would react in almost similar way with alkenes. For example, alkynes could react with hydrogen in addition reaction to form alkene.
B. Alkyl Halides (Haloalkanes)
Alkyl halides could be prepared from free radical substitution of alkanes, or addition reaction of alkenes with halogens or hydrogen halides.
Where X is halogen atom (F, Cl, Br, or I)
1. The reaction
a. The substitution reaction
Alkyl halide functional group could be substituted by another functional group. Therefore, it could react under the substitution reaction. The reaction has two types based on the mechanism of the reaction; nucleophilic substitution reaction unimolecular (SN1) and nucleophilic substitution reaction bimolecular (SN2).i. SN2 mechanism
For SN2 reaction mainly happen in the primary alkyl halide. The reaction is called bimolecular because the rate of the reaction depends on both alkyl halide and nucleophile. In the transition state, the leaving group is still attached but it also starts to brake and the nucleophile is starting to form the covalent bond with the carbon atom. This process of moving out of the leaving group and creating the bond of nucleophile are in the same moment. Therefore the mechanism is:
From the mechanism above, this mechanism does not have an intermediate as the cleavage of C-Br bond and the formation of C-OH happens at the same time which is called concerted reaction. The consequences of this mechanism are the nucleophile will react faster with less hindered alkyl halide and if the starting material is enantiomeric pure, the product will have the inverse configuration of the starting material.
The relative rate of SN2 reaction can be summarised as:
ii. SN1 mechanism
It is called SN1 reaction because of the rate of reaction in this mechanism only depends on the alkyl halide, concentration of nucleophile has no effect. In this mechanism, the leaving group moves out first and then followed by formation of covalent bond by nucleophile. This is caused by the geometrical arrangement is sterically hindered, so there is not enough room for nucleophile to attack. Therefore, the mechanism is:
The formation of carbocation has several consequences. Firstly, it would react faster with more substituted carbocation (i.e. tertiary carbocation). Secondly, if the starting material is enantiomeric pure, the product would roughly a racemic mixture due to lost of stereocentre in intermediater. In addition, the series of relative rate in SN1 reaction is:
To summarise the substitution reaction:
b. Other reactions
i. Reaction with AgNO3
AgX is insoluble precipitate
ii. Elimination
C. Alcohols
Alcohols are the organic compounds that have –OH functional group. Because it has –OH functional group, so alcohols have relatively higher boiling point than the others organic compound due formation of hydrogen bond. Alcohols could be classified from the position of –OH functional group.Primary alcohols
Secondary alcohols
Tertiary alcohols
Alcohols could be formed mainly in three ways. Firstly, with substitution reaction of alkyl halide with OH- or with water.
Besides that, alcohols could also be synthesised from reaction of alkene with steam and acid catalyst. Lastly, in the main industry of alcohols, it could be made from fermentation reaction.
a. Reaction
i. Combustion
Since alcohol is one of the types of fuel, so the main reaction is combustion reaction. Alcohols could react in combustion in the same way with other fuel, complete combustion and incomplete combustion.
ii. Substitution
OH functional group is one of the types leaving group, so alcohols could have substitution reaction. However, the substitution of alcohols should be helped with acid catalyst to protonate the alcohols, so it would be easier to leave.
This reaction also could be used for identification of alcohol’s classification. The reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid; however it can be used for low molecular weight only. This reagent is called Lucas’ reagent and the observation is the relative rate. For example:
- No visible reaction: primary, e.g. pentan-1-ol.
- Solution turns cloudy in 3-5 minutes: secondary, e.g. pentan-2-ol
- Solution turns cloudy immediately, and/or phases separate: tertiary, e.g. 2-methyl butan-2-ol
iii. Elimination reaction
Besides it can undergo substitution reaction, alcohols could undergo elimination reaction as well to form alkene.
iv. Esterification
The mechanism will be discussed later in the section about carboxylic acid derivates.
v. Identification reactions
1) Reaction with alkaline aqueous iodineThis reaction has a function to identify CH3CH(OH)– group.
2) Reaction with sodium metal
Alcohols also have acid properties, but it is very weak acid (pKa ethanol = 15.1). Therefore, alcohol still can react with Na metal but not with NaOH.
3) Oxidation reaction (Reaction with acidified K2Cr2O7)
Comments