Unit 7: Reactions of Hydrocarbons
Exploring the characteristic chemical reactions of alkanes, alkenes, and alkynes.
7.20 Alkanes & Haloalkanes (Preparation, Uses, Substitution)
Alkanes are saturated hydrocarbons, meaning they only contain C-C and C-H single bonds. This makes them relatively unreactive. Their most important reaction is combustion, but they can also undergo substitution reactions.
Free-Radical Substitution:
In the presence of ultraviolet (UV) light, alkanes react with halogens (like $Cl_2$ or $Br_2$) in a substitution reaction, where a hydrogen atom is replaced by a halogen atom. This reaction proceeds via a free-radical mechanism.
Example: Chlorination of methane.
$CH_4(g) + Cl_2(g) \xrightarrow{UV light} CH_3Cl(g) + HCl(g)$
The product, chloromethane ($CH_3Cl$), is a haloalkane. The reaction
can continue, leading to a mixture of products ($CH_2Cl_2$, $CHCl_3$, $CCl_4$).
Haloalkanes are useful as solvents and as intermediates in the synthesis of many other organic compounds.
Solved Examples:
- What is a substitution reaction?
Solution: A reaction in which an atom or group of atoms is replaced by another atom or group of atoms. - What condition is required for the reaction between an alkane and a
halogen?
Solution: The presence of ultraviolet (UV) light. - Write the equation for the first substitution reaction between ethane
($C_2H_6$) and bromine.
Solution: $C_2H_6(g) + Br_2(g) \xrightarrow{UV light} C_2H_5Br(g) + HBr(g)$. - What is the name of the product
$C_2H_5Br$?
Solution: Bromoethane. - Why are alkanes described as
"saturated"?
Solution: Because they contain only single bonds and cannot undergo addition reactions; their carbon atoms are saturated with hydrogen atoms. - Why is the free-radical substitution of alkanes not a good method for
preparing a pure haloalkane?
Solution: It produces a mixture of products due to further substitution and substitution at different positions on longer chains. - What class of compound is formed when an alkane reacts with a
halogen?
Solution: A haloalkane. - What is the role of UV light in this
reaction?
Solution: It provides the energy to break the halogen-halogen bond (e.g., $Cl-Cl$) to initiate the reaction by forming free radicals. - Name the product formed when chloromethane reacts further with
chlorine.
Solution: Dichloromethane ($CH_2Cl_2$). - Are alkanes generally reactive or unreactive?
Why?
Solution: Unreactive, because the C-C and C-H single bonds are strong and non-polar.
7.21 Alkenes (Preparation, Addition, Tests)
Alkenes are unsaturated hydrocarbons containing at least one C=C double bond. The presence of the electron-rich pi-bond makes them much more reactive than alkanes. Their characteristic reaction is electrophilic addition.
Preparation of Alkenes:
Alkenes are typically prepared in the lab by the elimination (dehydration) of an alcohol, using a hot catalyst like concentrated sulphuric acid or aluminium oxide.
Example: Dehydration of ethanol.
$CH_3CH_2OH(g) \xrightarrow{Al_2O_3, heat} CH_2=CH_2(g) + H_2O(l)$
Addition Reactions of Alkenes:
In an addition reaction, the double bond breaks, and atoms are added across the two carbon atoms.
- Addition of Hydrogen (Hydrogenation): Alkene + $H_2$ $\xrightarrow{Ni catalyst, heat}$ Alkane. (e.g., Ethene → Ethane)
- Addition of Halogens (Halogenation): Alkene + $X_2$ → Dihaloalkane. (e.g., Propene + $Br_2$ → 1,2-dibromopropane)
- Addition of Hydrogen Halides: Alkene + $HX$ → Haloalkane. (e.g., Ethene + $HBr$ → Bromoethane)
- Addition of Water (Hydration): Alkene + $H_2O(g)$ $\xrightarrow{H_3PO_4 catalyst, heat}$ Alcohol. (e.g., Ethene → Ethanol)
Test for Unsaturation (Alkenes):
The addition reaction with bromine is used as a chemical test. When an alkene is shaken with bromine water (an orange/brown solution), the alkene undergoes an addition reaction, and the solution is rapidly decolourised.
Solved Examples:
- What is the characteristic reaction of
alkenes?
Solution: Electrophilic addition. - What is the product when propene reacts with hydrogen gas in the presence of
a nickel catalyst?
Solution: Propane. - Describe the chemical test for an
alkene.
Solution: Add a few drops of bromine water and shake. A positive test is the rapid decolourisation of the orange/brown solution. - What is the name of the product when ethene reacts with chlorine
($Cl_2$)?
Solution: 1,2-dichloroethane. - What type of reaction is the conversion of ethanol to
ethene?
Solution: An elimination or dehydration reaction. - Why are alkenes described as
"unsaturated"?
Solution: Because they contain a C=C double bond and can undergo addition reactions to bond with more atoms. - What are the reagents and conditions needed to hydrate ethene to form
ethanol?
Solution: Steam ($H_2O(g)$), high temperature and pressure, and a solid phosphoric(V) acid catalyst. - Write the equation for the reaction of but-1-ene with bromine
water.
Solution: $CH_2=CHCH_2CH_3 + Br_2 \rightarrow CH_2BrCHBrCH_2CH_3$. - What is the product when propene reacts with
HBr?
Solution: The major product is 2-bromopropane (due to Markovnikov's rule), but 1-bromopropane is also formed. - Would an alkane decolourise bromine
water?
Solution: No, not under normal lab conditions. It would only react in the presence of UV light in a slow substitution reaction.
7.22 Alkynes (Preparation, Reactions, Tests)
Alkynes are unsaturated hydrocarbons containing at least one C≡C triple bond. The most common alkyne is ethyne ($C_2H_2$), also known as acetylene.
Preparation and Uses of Ethyne:
Ethyne is produced industrially from the cracking of larger hydrocarbons or by the reaction
of calcium carbide with water.
$CaC_2(s) + 2H_2O(l) \rightarrow C_2H_2(g) + Ca(OH)_2(aq)$
It is used in oxy-acetylene torches for welding and cutting metals, and as a starting
material for making polymers like PVC.
Reactions of Alkynes:
Like alkenes, alkynes are unsaturated and undergo addition reactions. The triple bond can react with one or two moles of the reagent.
Example: Reaction of ethyne with bromine.
- Step 1: $CH≡CH + Br_2 \rightarrow CHBr=CHBr$ (1,2-dibromoethene)
- Step 2: $CHBr=CHBr + Br_2 \rightarrow CHBr_2CHBr_2$ (1,1,2,2-tetrabromoethane)
Tests for Alkynes:
Alkynes will decolourise bromine water, just like alkenes. However, terminal alkynes (where the triple bond is at the end of the chain) can be distinguished by their reaction with ammoniacal silver nitrate (Tollens' reagent) or ammoniacal copper(I) chloride, where they form a solid precipitate (a silver or copper acetylide).
Solved Examples:
- What is the functional group of an
alkyne?
Solution: A carbon-carbon triple bond (C≡C). - What is the common name for ethyne?
Solution: Acetylene. - Write the equation for the complete hydrogenation of propyne
($CH_3C≡CH$).
Solution: $CH_3C≡CH + 2H_2 \xrightarrow{Ni catalyst} CH_3CH_2CH_3$. - How many moles of $HBr$ can react with one mole of
but-1-yne?
Solution: Two moles. - What is the industrial use of ethyne that takes advantage of its highly
exothermic combustion?
Solution: Oxy-acetylene welding and cutting torches. - Do alkynes decolourise bromine water?
Solution: Yes, because they are unsaturated and undergo addition reactions. - What is a terminal alkyne?
Solution: An alkyne where the triple bond is at the end of the carbon chain (involves C1). - How can you chemically distinguish between but-1-ene and
but-1-yne?
Solution: Add Tollens' reagent (ammoniacal silver nitrate). But-1-yne (a terminal alkyne) will give a white precipitate of silver acetylide, while but-1-ene will show no reaction. - What is the product when ethyne reacts with one mole of
HCl?
Solution: Chloroethene ($CH_2=CHCl$). - Are alkynes saturated or unsaturated?
Solution: Unsaturated.