Alkene structure isomerism and reactions

Alkene Structure and Bonding

Alkenes are unsaturated hydrocarbons containing a carbon–carbon double bond, with the general formula CₙH₂ₙ for non-cyclic alkenes with one C=C.

A cycloalkene is also unsaturated because it contains one C=C inside the ring, but its general formula is CₙH₂ₙ₋₂ because the ring closure removes two hydrogen atoms.

The C=C double bond is made of two different bonds working together. The first is a σ bond, formed by the head-on overlap of two sp² orbitals along the line joining the two carbon atoms. The second is a π bond, formed by the sideways overlap of two unhybridised p-orbitals, one on each carbon atom, above and below the molecular plane.

The π bond is weaker than the σ bond because sideways overlap is less efficient than head-on overlap, so the π electrons are exposed and electron-rich. This is why alkenes are reactive: the π electrons attract electrophiles, which is the basis of every addition reaction in this topic.

The restricted rotation around the C=C arises because rotating one carbon would break the π bond, which requires significant energy.

Geometric Isomerism in Alkenes

Geometric isomerism is a type of stereoisomerism that arises because of restricted rotation around the C=C double bond combined with two different groups on each double-bond carbon.

Rotation around a C–C single bond is free because only the σ bond connects the carbons. Rotation around a C=C double bond would require breaking the π bond, so rotation is restricted. This locks the substituents in fixed positions either side of the double bond, producing two distinct molecules with the same structural formula but different spatial arrangements.

For geometric isomerism to exist, each carbon of the C=C must carry two different groups. If either carbon has two identical groups, no geometric isomers exist because flipping the molecule produces the same compound.

• But-2-ene (CH₃CH=CHCH₃) shows geometric isomerism: each double-bond carbon has one H and one CH₃, so the two methyl groups can lie on the same side or opposite sides.
• But-1-ene (CH₂=CHCH₂CH₃) does not show geometric isomerism because one of the double-bond carbons has two hydrogen atoms.

E–Z Naming System

The E–Z system assigns priority to the two groups on each double-bond carbon using atomic number and is used when the cis–trans system cannot give an unambiguous name.

Each carbon of the C=C is examined separately. The atom directly bonded to the C=C carbon is compared, and the higher atomic number takes priority. If the priorities tie, the next set of atoms outwards is compared.

If the two higher-priority groups lie on the same side of the C=C, the isomer is labelled Z (from the German zusammen, meaning “together”). If they lie on opposite sides, it is labelled E (from entgegen, meaning “opposite”).

The cis–trans system breaks down when each double-bond carbon carries three or four different groups, because there is no longer a single “main chain” to label as cis or trans. For example, 1-bromo-1-chloro-2-fluoroethene has four different substituents on the C=C, so cis–trans cannot describe it. E–Z always works because priority depends only on atomic number.

Examiner InsightThe E–Z system uses atomic number, not size or mass. Br > Cl > F > O > N > C > H. State priorities explicitly when justifying an E or Z label.

Exam TipWrite “Br has a higher atomic number than Cl, so Br has higher priority on this carbon” rather than “Br is bigger”.

Addition Reactions of Alkenes

Alkenes undergo addition reactions because the electron-rich π bond attracts electrophiles or reacts with reducing/oxidising agents, breaking the π bond and forming two new σ bonds.

The five reactions required by the syllabus are summarised below. Each converts the C=C into a single bond and adds new atoms to the two carbons.

The hydrogenation of alkenes converts vegetable oils into solid fats by saturating the C=C bonds, which is the chemistry behind margarine production.

The addition of acidified potassium manganate(VII) oxidises the C=C and adds an –OH group to each carbon. The purple MnO₄⁻ is decolourised, which can also serve as a test for unsaturation.

For unsymmetrical alkenes such as propene, addition of HBr can give two possible products, and the major product is determined by carbocation stability (covered in the mechanism subtopic).

MisconceptionStudents often write “Pt catalyst” for hydrogenation of alkenes. Edexcel specifies a nickel catalyst for the alkene-to-alkane reaction. Pt and Pd work but are not the named catalyst.

Exam TipWrite Ni as the catalyst unless the question states otherwise.

Test for the C=C Double Bond

The bromine test identifies a C=C double bond: shaking a sample with orange bromine water decolourises the solution if the molecule is unsaturated.

The orange colour of Br₂ disappears because the bromine adds across the C=C to form a colourless dibromoalkane. A saturated compound such as an alkane has no C=C and produces no decolourisation in the absence of UV light.

Bromine in an inert organic solvent (orange–brown) and bromine water (orange) both work. Bromine water adds H–OH and Br–Br across the double bond to give a halohydrin and HBr, but the diagnostic feature is the same: decolourisation.

SafetyBromine and bromine water are toxic and corrosive. Use a fume cupboard and wear gloves and goggles.

Electrophilic Addition Mechanisms

The reactions of alkenes with Br₂ and HBr proceed by electrophilic addition, where the electron-rich π bond attacks an electrophile, forming a carbocation intermediate that is then attacked by a nucleophile.

For HBr, the H–Br bond is permanently polar (Hᵟ⁺–Brᵟ⁻), so the H atom acts directly as the electrophile. For Br₂, the molecule is non-polar but becomes polarised on approach to the π bond — an induced dipole is generated as the π electrons repel the bonding electrons in Br₂.

In step 1, the π electrons attack the δ⁺ atom (H of HBr or the closer Br of Br₂). The π bond breaks heterolytically, both electrons going to form the new σ bond, and a positively charged carbocation intermediate is formed on the other carbon.

In step 2, the leaving group (Br⁻) attacks the carbocation using a lone pair, forming the second new σ bond and giving the neutral product.

Carbocation stability follows the order tertiary > secondary > primary. Alkyl groups donate electron density (positive inductive effect, +I) onto the positively charged carbon, stabilising it. More alkyl groups means more stabilisation.

For propene + HBr, the H can attach to either C1 or C2 of the C=C. Attaching H to C1 places the positive charge on C2, giving a secondary carbocation; attaching H to C2 places the positive charge on C1, giving a primary carbocation.

The secondary carbocation is more stable, so it forms more readily. Br⁻ then attaches to C2, giving 2-bromopropane as the major product. This is Markovnikov addition: H goes to the carbon already bearing more H atoms.

Addition Polymerisation of Alkenes

Many alkene molecules join together by addition polymerisation, where the C=C bonds open and the monomers link to form long chain polymers with no other product released.

The repeat unit is shown in square brackets with a single bond on each side and the subscript n outside the brackets, indicating that the unit repeats many times. The C=C of the monomer becomes a C–C single bond in the polymer.

• For ethene, n CH₂=CH₂ → –[CH₂–CH₂]ₙ–, giving poly(ethene).
• For propene, n CH₂=CHCH₃ → –[CH₂–CH(CH₃)]ₙ–, giving poly(propene).
• For chloroethene, n CH₂=CHCl → –[CH₂–CHCl]ₙ–, giving poly(chloroethene), known as PVC.

To draw the repeat unit from a given monomer, change the C=C to a C–C single bond, place the atoms or groups in the same orientation as on the monomer, and add a bond extending from each end of the unit out through the brackets.

To draw the monomer from a given repeat unit, take the two carbons inside the brackets, restore the C=C double bond, and remove the bonds extending through the brackets.

Polymer Disposal and Environmental Solutions

Disposing of waste polymers is an environmental problem because most addition polymers are non-biodegradable, with stable C–C and C–H bonds resistant to microbial breakdown.

Two strategies named in the syllabus are used to limit these problems.

The first is the development of biodegradable polymers that microorganisms can break down. These often contain ester or other hydrolysable linkages incorporated into the chain, so that hydrolysis by soil bacteria converts the polymer back into simple molecules. Examples include poly(lactic acid) and starch-based polymers. These reduce the volume of plastic waste in landfill and lower the persistence of plastic in the environment.

The second is the removal of toxic gases produced when polymers are incinerated. Burning poly(chloroethene) (PVC) releases HCl, while incomplete combustion of any polymer can produce CO and other harmful products. These gases are removed by passing the flue gases through scrubbers containing alkaline solutions (e.g. CaO or Ca(OH)₂), which neutralise the HCl, and by using high-temperature combustion with adequate oxygen to convert CO into CO₂.

Incineration also recovers energy from the combustion as heat or electricity, reducing reliance on fossil fuels but raising concerns over CO₂ emissions and toxic by-products.