C11 Alkenes

Question 1

What are alkenes? What is the general formula of an alkene?

Answer 1

Alkenes are hydrocarbons that contain one or more carbon-carbon double bond. The functional group of an alkene is the C=C bond. Non-cyclic alkenes that contain one C=C bond per molecule form a homologous series of unsaturated aliphatic hydrocarbons with the general formula CnH2n, where n is an integer greater than or equal to 2.

Question 2

What is a terminal alkene?

Answer 2

A terminal alkene contains the =CH2 structural unit where the C=C bond is located at one end of the carbon chain.

Question 3

What is a cycloalkene?

Answer 3

A cycloalkene contains one or more C=C bond in a ring structure.

Question 4

Explain how to name alkenes.

Answer 4

Pg 3 of notes

Identify the longest carbon chain that contains the C=C bond. Number the carbon atoms in the chain in the direction that gives the doubly bonded carbon atoms the lowest possible numbers. Specify the numerical position of the C=C bond by placing the number before the -ene suffix.

Alkenes with two C=C bonds are named as dienes and alkenes with three C=C bonds are named as trienes.

Question 5

Explain why the strength of the C=C bond is less than twice that of the C-C bond.

Answer 5

The bond energy of the C=C bond is greater than the bond energy of the C-C bond. However, the strength of the C=C bond is less than twice that of the C-C bond. Since the side-on overlap of the p orbitals is less effective than the head-on overlap of the orbitals, the pi bond is weaker than the sigma bond.

Question 6

Explain how cis-trans isomerism can exist in alkenes.

Answer 6

Rotation about the C=C bond requires the breaking of the pi bond. Hence, there is restricted rotation about any C=C bond. This restricted rotation can give rise to cis-trans isomerism when each doubly bonded carbon atom is joined to two different atoms or groups.

Question 7

Explain why cyclohexene does not display cis-trans isomerism.

Answer 7

“trans-cyclohexene” is too unstable to exist at room temperature due to its highly strained structure.

Question 8

Explain why a cis alkene is less stable than its stereoisomeric trans alkene.

Answer 8

A cis alkene is less stable than its stereoisomeric trans alkene because of steric strain, due to repulsion between electron clouds, arising from crowding between the two alkyl groups in the cis-isomer. The difference in stability between cis-but-2-ene and trans-but-2-ene can be quantified by comparing their standard enthalpy change of combustion data.

Question 9

Describe and explain the trend of boiling and melting points of alkanes with an increasing number of carbon atoms.

Answer 9

The boiling and melting points of alkanes increase with increasing number of carbon aotms. Alkanes contain C-C, C-H and C=C bonds. Since the difference in electronegativity between the C and H atoms are negligible, the C-H bond and thus alkanes are essentially non-polar. The intermolecular forces of attraction between alkane molecules are relatively weak instantaneous dipole-induced dipole interactions. As the number of carbon atoms in alkanes increases, the number of electrons per alkane molecule increases and the size of the electron cloud of the alkane molecule increases. As a result, the ease of polarisation of the electron cloud increases leading to the increased strength of instantaneous dipole-induced dipole attractive forces which are overcome at increasingly higher temperatures.

Question 10

Explain the solubility of alkenes in various solvents.

Answer 10

Alkenes are insoluble in water but quite soluble in non-polar solvents.

Question 11

Describe and explain the trend of the density of alkenes.

Answer 11

With the increasing molecular size of the alkenes, the density of alkenes increases. All alkenes are less dense than water.

Question 12

Explain why the boiling point of cis-but-2-ene is higher than that of trans-but-2-ene.

Answer 12

Cis-but-2-ene is slightly polar. There exists instantaneous dipole-induced dipole (id-id) and permanent dipole-permanent dipole attractive (pd-pd) forces between the cis-but-2-ene molecules. However, trans-but-2-ene is non-polar and only id-id interactions exist between the molecules. Since more energy is required to overcome the stronger intermolecular forces present in cis-but-2-ene than in trans-but-2-ene, cis-but-2-ene has a higher boiling point than trans-but-2-ene.

Question 13

Explain why the melting point of cis-but-2-ene is lower than that of trans-but-2-ene.

Answer 13

The molecules of cis-but-2-ene pack poorly in the solid lattice because the two bulky methyl groups are located on the same side of the molecule. The molecule is less symmetrical and the poor packing results in larger distances between the molecules, which lead to weaker intermolecular forces and hence a lower melting point for cis-but-2-ene.

Question 14

In which direction does the arrow point in polar molecules?

Answer 14

The arrow points towards the more electronegative atom.

Question 15

Why are alkenes highly reactive compounds in comparison with alkanes?

Answer 15

The high reactivity of alkenes is attributed to the presence of the electron-rich C=C bond. The pi electrons in the C=C bond act as a source of electrons during many reactions and they attract reagents known as electrophiles (species that like electrons).

Question 16

Explain why alkenes undergo electrophilic addition reactions.

Answer 16

Since the pi bond is much weaker than the sigma bond in a C=C bond, it is much more easily broken. As a result, alkenes undergo addition reactions in which the pi bond is broken and two new sigma bonds are formed. This reaction only occurs when there is unsaturation in the reactant molecules. During the reaction, 2 species react to give a single product, there is a decrease in unsaturation and the sigma bond of the unsaturated bond remains unbroken. Since alkenes attract electrophiles and undergo addition reactions, the characteristic reactions of alkenes are termed electrophilic addition reactions.

Question 17

State the 5 types of electrophilic addition reactions of alkenes.

Answer 17

1) Addition of hydrogen halides (HX)
2) Addition of water (laboratory method)
3) Addition of steam (industrial method)
4) Addition of halogens (X2) in CCl4
5) Addition of halogens (X2) in H2O (water)

Question 18

State the two types of oxidation reactions of alkenes. Do alkenes undergo oxidation with potassium dichromate?

Answer 18

1) Mild oxidation in cold, alkaline medium
2) Mild oxidation in cold, acidic medium
3) Strong oxidation in hot, acidic medium
4) Strong oxidation in hot, alkaline medium

They do not undergo oxidation with K2Cr2O7 (aq), H2SO4 (aq) (hot, acidified potassium dichromate).

Question 19

State the two types of reduction reactions of alkenes. State and explain the types of reducing agents which alkenes do not react with.

Answer 19

1) Addition of H2 using Ni catalyst (with use of heat)
2) Addition of H2 using Pt/Pd catalyst (without use of heat)

They do not react with reducing agents such as LiAlH4 and NaBH4 because the sp2 hybridised C atoms of the C=C group in alkenes do not have a partial positive charge and hence do not attract nucleophiles such asLiAlH4 (lithium aluminium hydride), NaBH4, HCN (hydrogen cyanide).

Question 20

State Markovnikov’s rule.

Answer 20

In the addition of a hydrogen halide (HX) to the C=C bond of an unsymmetrical alkene, the major product is the one in which the hydrogen atom of the HX attaches itself to the double bonded carbon atom already bonded with the greater number of hydrogen atoms. (However, this is only predictive and we need to take a closer look at the mechanism for the reaction between an unsymmetrical alkene and a hydrogen halide.The mechanistic explanation of why one product predominates over the other possible product lies in the relative energetic stabilities of the carbocation intermediates formed during the reaction.)

Question 21

What is a carbocation?

Answer 21

A carbocation is an ion with a positively charged carbon. It is extremely reactive and has a strong tendency to accept a pair of electrons.

Question 22

Explain the trend in the rate of formation of primary, secondary, tertiary carbocations.

Answer 22

Pg 14 of notes

A carbocation can be classified into primary, secondary and tertiary based on the number of alkyl groups bonded to the carbon atom bearing the positive charge.

The stability of the charged system is increased by the dispersal of the charge (Therefore, any factor that tends to spread out the positive charge of the electron-deficient carbon and distribute it over the rest of the ion will stabilise the carbocation. Groups that are bonded to the carbon bearing the positive charge can be electron-donating - disperses the positive charge on the carbon - CH3, CH2CH3, stabilising the carbon or electron-withdrawing - intensifies the positive charge on the carbon, destabilising the carbocation - NO2, COOH.)

Alkyl groups are electron-donating. The greater the number of electron-donating groups attached to the carbon bearing the positive charge, the greater the dispersal of the positive charge and hence the greater the stability of the carbocation. Hence, the stability of the carbocation increases from methyl carbocation -> primary carbocation -> secondary carbocation -> tertiary carbocation. The more stable a carbocation, the lower is the activation energy required for its formation and hence the faster its rate of formation. Hence the rate of formation increases from methyl carbocation -> primary carbocation -> secondary carbocation -> tertiary carbocation.

Question 23

State the Mechanism-based Markovnikov’s rule.

Answer 23

In an electrophilic addition reaction, the electrophile adds to an unsymmetrical alkene to produce the more stable carbocation intermediate. Being formed faster in the slow step, the more stable the carbocation is more available to participate in the fast step to give the major product.

Question 24

Describe the reaction of alkenes with hydrogen halides. State the reagent, conditions and mechanism of the reaction.

Answer 24

The reaction involves the addition of H and X atoms of a hydrogen halide across the C=C bond of the alkene to form a halogenoalkane or alkyl halide.

Reagent: HX (g) where X = Cl, Br or I
Conditions: dry HX (g) and room temperature
Mechanism: Electrophilic addition

Question 25

Explain why dry gaseous hydrogen halides must be used during the reaction of alkenes with hydrogen halides.

Answer 25

Dry gaseous hydrogen halides must be used during the reaction of alkenes with hydrogen halides as aqueous solutions of hydrogen halides such as hydrochloric acid cannot be used for this reaction because it will result in the formation of alcohols (Recall: Reaction of alkenes with halogens in H2O).

Question 26

Why are major and minor products formed during reactions involving alkenes?

Answer 26

Unlike symmetrical alkenes, in unsymmetrical alkenes, reactions can result in the formation of two isomeric products. They are not formed in equal amounts. The product formed in greater amount is termed the ‘major product’ and the product formed in smaller amount is termed the ‘minor product’.

Question 27

Describe and draw the electrophilic addition mechanism.

Answer 27

Refer to handwritten notes for mechanisms

Hint: Electrophilic addition mechanism consists of two steps: 1) Addition of electrophile and 2) Nucleophilic Attack.

Question 28

Describe the reaction of alkenes with water. State the reagent, conditions and mechanism of the reaction.

Answer 28

Refer to handwritten notes for mechanisms

Electrophilic addition of water to alkenes
(Laboratory method)

Alkenes do not react with water directly. In the laboratory, the addition of H2O across the C=C bond of an alkene to form alcohol can be carried out via a two-step process: 1) Electrophilic addition and 2) Hydrolysis.

Step 1: cold conc H2SO4
Step 2: H2O, heat

In the first step of the reaction, the alkene reacts with cold concentrated H2SO4 through electrophilic addition to form an alkyl hydrogensulfate. In the second step of the reaction, the alkyl hydrogensulfate is heated with water and is being converted to alcohol through hydrolysis where the covalent bond is cleaved by reaction with water.

Question 29

Describe one simple chemical test which would enable you to distinguish between pent-1-ene and pent-2-ene.

Answer 29

Add two drops of KMnO4 acidified with dilute sulfuric acid to each sample in a test tube, and heat. For Pent-2-ene, there is decolourisation of purple KMnO4 and no gas evolved. For Pent-1-ene, there is decolourisation of purple KMnO4 and the evolution of a colorless gas (CO2) which gives a white precipitate with lime water.

(Alkenes tutorial Q4b)

Question 30

Consider a substituent, G, bonded to a positively charged carbon. Compared to a hydrogen atom, G may be either electron-donating or electron-withdrawing. Explain the effects of electron-donating groups and electron-withdrawing groups on the carbocation.

Answer 30

If G is an electron-donating group, it disperses the positive charge on the carbon and stabilised the carbocation. If G is electron-withdrawing, it intensifies the positive charge in the carbon and destabilizes the carbocation.

Question 31

Describe the reaction of alkenes with steam. State the reagent, conditions and mechanism of the reaction.

Answer 31

Refer to handwritten notes for mechanisms

Electrophilic addition of steam to alkenes
(Industrial method)

Reagent: H2O (g)
Conditions: concentrated H3PO4 (catalyst - phosphoric acid) on silica support under high pressure and temperature
Mechanism: Electrophilic addition

Question 32

Describe the reaction of alkenes with halogens in CCl4. State the reagent, conditions and mechanism of the reaction.

Answer 32

Refer to handwritten notes for mechanisms

Electrophilic addition of halogens to alkenes (in CCl4))

Reagent: X2(g) dissolved in an inert solvent (CCl4 or CH2Cl2) where X = Br, Cl
Conditions: dark (absence of ultra-violet light) to prevent FRS from occurring), room temperature
Mechanism: Electrophilic addition

The addition of a halogen X2 to an alkene produces a vicinal dihalide (an organic compound in which the 2 halogen atoms are on adjacent carbon atoms)

Step 1: Addition of electrophile
Step 2: Nucleophilic attack by Br-

Question 33

What is a vicinal dihalide?

Answer 33

An organic compound in which the 2 halogen atoms are on adjacent carbon atoms)

Question 34

Explain why F2 and I2 are seldom used for the halogenation of alkenes.

Answer 34

Br2 and Cl2 are the two halogens that are normally used in halogen addition to an alkene. F2 and I2 are seldom used as F2 react explosively with alkenes while the addition of I2 to an alkene is a thermodynamically unfavourable reaction. The vicinal diiodides are unstable at room temperature and decompose back to the corresponding alkenes and I2.

Question 35

What does the amount of halogen reacting with a mole of alkene tells us about the alkene molecule?

Answer 35

The amount of Br2 reacted per mole of alkene is a measure of the number of C=C bonds present in the alkene molecule. One mole of C=C reacts with one mole of Br2.

Question 36

Describe the reaction of alkenes with halogens in H2O. State the reagent, conditions and mechanism of the reaction.

Answer 36

Refer to handwritten notes for mechanisms

Electrophilic addition of halogen to alkenes (in H2O)

Reagent: X2 (g) in water or X2 (aq) or X2/H2O where X = Cl, Br and I
Conditions: dark, room temperature
Mechanism: Electrophilic addition

Step 1: Addition of electrophile
Step 2: Nucleophilic attack by H2O (in excess)
(Note: In step 2, the nucleophiles, with lone pairs to act as electron donors, available are Br- and H2O. Since H2O is in excess, the nucleophilic attack is by H2O to form an oxonium ion. Alternative step 2 is nucleophilic attack by Br-)
Step 3: Loss of proton (by oxonium ion)

Question 37

During the reaction of alkenes with halogens in H2O, can a diol be formed?

Answer 37

No, a diol cannot be formed as the electrophile present is the partial positive Br atom. In other words, a Br containing carbocation is formed.

Question 38

Describe the reaction of alkenes with cold, alkaline potassium manganate (VII). State the reagent, conditions, type of the reaction and the observations made.

Answer 38

Alkenes undergo mild oxidation with cold, alkaline potassium manganate (VII) to form diols. This reaction involves the addition of two -OH groups across the C=C bond of an alkene.

Reagents: KMnO4(aq), NaOH(aq)
Conditions: cold
Type of reaction: Oxidation
Observations: Purple KMnO4 is decolourised and a brown-black ppt of MnO2 is observed.

The reaction occurs without complete cleavage of the C=C bond and is regarded as mild oxidation of an alkene. (Note: Cold acidified KMnO4 is less commonly used.)

Question 39

Describe the reaction of alkenes with hot acidified potassium manganate (VII). State the reagent, conditions, type of the reaction and the observations made.

Answer 39

Alkenes undergo strong oxidation with hot, acidic potassium manganate (VII) to form ketones, carboxylic acids and/or CO2 gas. This reaction is also called the oxidative cleavage as it takes place with the complete cleavage of the C=C bond.

Reagents: KMnO4(aq), H2SO4(aq)
Conditions: heat/heat under reflux
Type of reaction: Oxidation
Observations: Purple KMnO4 is decolourised and a brown-black ppt of MnO2 is observed. For terminal alkenes (=CH2), a colourless gas CO2 which forms a white ppt with limewater is evolved (carbonic acid formed readily decomposes to give CO2 and H2O).

The degree of substitution on the C=C bond of the alkene determines the oxidation products obtained. In general, the C=C is cleaved and becomes C=O, while the H atoms bonded to the C atoms are replaced with -OH.

Question 40

Describe the reaction of alkenes with hot alkaline potassium manganate (VII). State the reagent, conditions, type of the reaction and the observations made.

Answer 40

Reagents: KMnO4(aq), NaOH(aq)
Conditions: heat/heat under reflux
Type of reaction: Oxidation
Observations: Purple KMnO4 is decolourised and a brown-black ppt of MnO2 is observed.

In general, the products of this reaction are those of the reaction of alkenes with hot, acidic KMnO4 but the OH group becomes O-Na+ and terminal alkenes form Na2Co3 + H2O instead.

Question 41

Describe the reduction of alkenes. State the reagent, conditions, type of the reaction and mechanism.

Answer 41

Alkenes undergo reduction when heated with hydrogen gas in the presence of nickel catalyst to give alkanes. The overall reaction involves the addition of hydrogen atoms across the C=C bond. (Note: The number of moles of H2 needed to reduce the molecule can tell us about the number of C=C bonds in the molecule.)

Reagent: H2(g)
Conditions: Ni catalyst, heat or Pt catalyst or Pd catalyst
Type of reaction: Reduction
Mechanism: Refer to heterogeneous catalysis in kinetics lecture notes

Question 42

What is catalytic hydrogenation?

Answer 42

It is the reduction of an alkene to an alkane by hydrogen in the presence of a catalyst.

Question 43

State the general balanced equation of the combustion of alkenes.

Answer 43

Refer to handwritten notes

Question 44

Describe what addition polymers are.

Answer 44

Addition polymers are formed with monomer containing C=C double bonds (e.g. alkenes) by adding monomers to the growing end of a polymer chain.

Question 45

Describe the naming of polymers.

Answer 45

Polymers are usually named according to the monomers from which they are prepared - the polymer is named by enclosing the name of the monomer(s) in parentheses ( () ) and adding the prefix poly.

Question 46

State the empirical formula of a polymer.

Answer 46

The empirical formula of the polymer is the same as its monomer.

Question 47

Draw the structure of a polymer.

Answer 47

Enclose the repeat unit within the brackets, followed by the letter n to indicate the number of repeated units.

Question 48

State a simple chemical test that can differentiate between pentane and pent-2-ene and state the observations of the reaction.

Answer 48

Test: Add Br2 in CCl4 drop-wise to each sample in a test tube at room temperature.

Observations: For pent-2-ene, there is rapid decolourisation of the orange-red Br2.
For pentane, there is no rapid decolourisation of the orange-red Br2.

Question 49

State common chemical tests that are used to differentiate between alkenes and alkanes and their respective positive observations.

Answer 49

Adding Br2 in CCl4 to each sample in a test tube at room temperature -> rapid decolourisation of orange-red Br2 (must state negative of positive test too)

Add two drops of KMnO4(aq) acidified with dilute H2SO4 to each sample in a test tube and heat each reaction mixture in a hot water bath -> decolourisation of purple KMnO4 (Note: Never heat under reflux for SIMPLE chemical tests)

Add two drops of KMnO4(aq) and NaOH(aq) to each sample in a test tube under cold conditions -> purple KMnO4 decolourises, brown-black ppt of MnO2 observed for alkene but not for alkane

If the sample is a gas, bubbling a sample of each gas through Br2(aq) in separate test tubes at room temperature -> decolourisation of the orange-red Br2 (aq)

Question 50

Explain the process of ‘heating under reflux’ and what it is used for.

Answer 50

Heating under reflux is the process of heating a reaction mixture while continuously cooling the vapour produced via a Liebig condenser with running water. On heating, the volatile reactants/products vapourise. The vapour condenses on the inner walls of the cooled Liebig condenser and drips back into the flash as a liquid. Hence, the loss of volatile reactants or products is minimised.

Thus, it is commonly used in organic synthesis which involves heating a reaction mixture containing volatile reactants/products for an extended period of time.

Question 51

State the two reactions through which alkenes can be prepared and name the type of reactions.

Answer 51

Refer to handwritten notes

Alkenes can be prepared through the following elimination reactions - involves the removal of atoms or groups of atoms from adjacent carbon atoms of the reactant molecule leading to the creation of a new pi bond in the product molecule.

(a) Dehydrohalogenation of halogenoalkanes (or alkyl halides)
(b) Dehydration of alcohols

Question 52

Describe the dehydrohalogenation of halogenoalkanes/alkyl halides. State the reagent, conditions and the type of the reaction.

Answer 52

Dehydrohalogenation of halogenoalkanes/alkyl halides is an elimination reaction. During the reaction, the H and X atom attached to adjacent carbon atoms are removed and an alkene is formed. This elimination reaction is known as dehydrohalogenation since the elements of HX are eliminated from the halogenalkane molecule.

Reagent: KOH in ethanol
Conditions: heat/heat under reflux
Type of reaction: Elimination

Mechanism is not in the A-Level syllabus

Question 53

Describe the dehydration of alcohols. State the reagent, conditions and the mechanism of the reaction.

Answer 53

Alcohols can under dehydration to give alkenes and the reaction involves the elimination of H2O from the alcohol molecule. the H atom and the hydroxyl group (-OH) from adjacent carbon atoms in the alcohol molecule are eliminated as a molecule of water.

Reagent: excess conc. H2SO4, heat / Al2O3 / H3PO4
Conditions: heat
Type of reaction: Elimination

*Refer to pg 36 of notes for experiment set-up

Question 54

State Saytzeff’s Rule (or Zaitsev’s Rule)

Answer 54

State Saytzeff’s Rule (or Zaitsev’s Rule) states that:

For elimination reactions of alcohols or halogenoalkanes to alkenes, the preferred product is the alkene with the greater number of alkyl groups bonded to the doubly bonded carbon atoms i.e. the more substituted alkene. Thus, the more substituted the alkene, the more electron-donating alkyl substituents attached to its doubly bonded carbon atoms, the more stable the alkene. In general, the stability of an alkene increase in the order: unsubstituted < monosubstituted < disubstituted < trisubstituted < tetrasubstituted. (Refer to pg 37 of notes for detailed disubstituted order)