Topic 5 - Introduction To Alkenes ( Part 1)

Question 1

Define Alkenes

Answer 1

Unsaturated hydrocarbons with at least one carbon-carbon double bond. They are part of a homologous series with the general formula CnH2n.

Question 2

Define Cycloalknaes

Answer 2

A type of alknen hydrocarbon, where the carbon atoms are arranged ina closed ring.

Question 3

Define high electron densitiy

Answer 3

High electron density refers to an area where electrons are more concentrated or found in higher amounts in a molecule or atom. This often happens where there are lone pairs of electrons, multiple bonds, or areas with electronegative atoms, leading to a greater attraction for electrons.

High electron density often leads to increased reactivity, especially in nucleophilic reactions, where the electron-rich areas attack positively charged or electron-deficient regions.

Question 4

Define lone pairs

Answer 4

Lone pairs are pairs of electrons that are not involved in bonding and are found in the outermost electron shell of an atom. These electrons belong to a single atom and are not shared with or donated to other atoms to form chemical bonds.

Question 5

What are the key points of lone pairs?

Answer 5

Key Points:
Non-Bonding Electrons: Lone pairs do not participate in chemical bonding, unlike electrons that form covalent bonds between atoms.
Electron Configuration: They are typically found in atoms that have more than two electrons in their outer shell, like oxygen, nitrogen, and halogens.
Influence on Molecular Shape: Lone pairs can influence the geometry and shape of a molecule by repelling bonding pairs of electrons.

Example:
In water (H₂O), the oxygen atom has two lone pairs of electrons that are not involved in bonding with hydrogen atoms. These lone pairs affect the molecule’s bent shape.

Lone pairs are important because they contribute to the reactivity of molecules and can influence properties like polarity and the molecule’s overall shape.

Question 6

Define σ bond

Answer 6

A sigma bond is a type of covalent bond formed when two atoms share a pair of electrons, with the electron density concentrated along the line connecting the two nuclei. It is the strongest type of covalent bond and occurs when atomic orbitals overlap head-on.

Key Points:
Single Bond: A sigma bond is typically found in a single bond between two atoms.
Head-on Overlap: The bond forms from the direct overlap of orbitals (such as s-s, s-p, or p-p orbitals).
Sigma bonds are the foundation of most chemical bonds in molecules.

Question 7

Define π bond

Answer 7

A pi bond is a type of covalent bond formed when two atoms share electrons in an side-to-side overlap of p orbitals. This bond is typically found in double and triple bonds, in combination with a sigma bond.

Key Points:
Side-to-Side Overlap: The electron density in a pi bond is concentrated above and below the axis connecting the nuclei.
Weaker than Sigma Bonds: Pi bonds are weaker than sigma bonds due to the less effective overlap of the orbitals.
Occurs in Multiple Bonds: Pi bonds are always present in double and triple bonds along with a sigma bond.

Example: In ethylene (C₂H₄), the carbon atoms are connected by a double bond, which consists of one sigma bond and one pi bond.

Question 8

Discuss about the carbon double bond

Answer 8

Has an area of high electron density, making it susceptible to attack from electrophiles (species that are attracted to delta-areas). It consists of a nomral covalent sigma bond and a pie bond.

Question 9

What is the widely known test for alkenes? Explain why the solution turned colourless.

Answer 9

Bromine water is used to identify an alknene double bond and other unsaturated compounds. Alkenes cause bromine water to change colour from orange-brown to colourless. This is because the C=C bond can ‘open up’ to accept bromine atoms and thus become saturated.

Question 10

Explain stereoisomers

Answer 10

Stereoisomers are molecules that have the same molecular formula and bond connectivity but differ in the three-dimensional arrangement of their atoms in space.

Question 11

Define geometric isomerism

Answer 11

Geometric isomerism is a type of stereoisomerism where molecules with the same molecular formula have different spatial arrangements of atoms or groups around a double bond or a ring structure, resulting in distinct isomers.

Question 12

Define different spatial arrangements.

Answer 12

Different spatial arrangement refers to the way atoms or groups are arranged in three-dimensional space around a central point, such as a carbon atom. Even though the atoms might be connected in the same sequence (same molecular formula), their positions in space can vary, resulting in different structures or isomers.

Question 13

define E-Z isomerism

Answer 13

E-Z isomerism is a type of geometric isomerism that occurs due to the different spatial arrangement of substituent groups around a double bond or a ring structure. It is a more specific naming system used when the cis-trans system is not applicable, especially in cases where there are two different groups attached to each carbon of the double bond

Question 14

Discuss about the E isomer

Answer 14

E Isomer (Entgegen): The term E comes from the German word “entgegen,” meaning “opposite.” In the E isomer, the higher-priority substituent groups on each carbon of the double bond are positioned on opposite sides.

Question 15

What does the Z isomer mean in german and where does it have its functional group on the compound?

Answer 15

Z Isomer (Zusammen): The term Z comes from the German word “zusammen,” meaning “together.” In the Z isomer, the higher-priority substituent groups on each carbon of the double bond are positioned on the same side.

Question 16

Explain high-priority substituent group

Answer 16

Higher-priority substituent groups refer to the groups attached to a carbon atom in a molecule that are ranked according to the Cahn-Ingold-Prelog priority rules. These rules are used to determine the order of importance based on atomic numbers and the structure of the groups involved.
Key Points:
1. Atomic Number: The group with the atom of higher atomic number is given higher priority. For example, a group with a carbon atom attached to a chlorine atom (C-Cl) will have a higher priority than a group with a carbon atom attached to a hydrogen atom (C-H).
2. Atomic Mass and Complexity: If two groups contain the same type of atom, the one with the heavier atom or the more complex group (in terms of the number of bonds or atoms) is given higher priority.
3. Cahn-Ingold-Prelog Rules: These rules help determine priority by examining the atoms directly attached to the carbon, and if necessary, looking further at the atoms in the group to determine which one has the highest atomic number or mass.
Example:
* In a molecule with two substituents on a double-bonded carbon:
o A Cl atom (atomic number 17) is a higher priority than a H atom (atomic number 1).
o CH₃ (methyl group) has a lower priority than OH (hydroxyl group), because oxygen (atomic number 8) is higher than carbon (atomic number 6).
The priority of substituent groups is important for naming E-Z isomers and determining the spatial arrangement of atoms around double bonds.

Question 17

Explain what is Cahn-Ingold-Prelong (CIP) priority rules

Answer 17

The Cahn-Ingold-Prelog (CIP) priority rules are a set of guidelines used to assign priorities to atoms or groups attached to a chiral center or a double bond, based on their atomic numbers and structure. These rules are essential for determining the configuration of molecules, especially when dealing with stereoisomers like E-Z isomers or R-S isomers.

Question 18

What are cis-isomers

Answer 18

A cis-isomer is a type of geometric isomer in which two identical or similar groups are positioned on the same side of a double bond or a ring structure. This term is commonly used when discussing cis-trans isomerism, where molecules with the same molecular formula differ in the arrangement of atoms or groups in space.

Key Points:
Same Side Arrangement: In a cis-isomer, the identical or similar groups are located on the same side of the double bond or ring structure.
Common in Alkenes and Cyclic Compounds: Cis-isomerism is frequently observed in alkenes (molecules with a carbon-carbon double bond) and cyclic compounds.
Physical Properties: Cis-isomers often have different physical and chemical properties from their corresponding trans-isomers (where groups are on opposite sides), such as melting and boiling points.
Example:
Cis-but-2-ene: In this molecule, the two methyl groups (CH₃) are on the same side of the double bond between the two carbon atoms.
Cis-2-butene has a bent shape due to the positioning of the groups, affecting its properties like polarity.
The term cis helps describe the relative positions of substituent groups, which is important for understanding the molecule’s behavior and reactivity.

Question 19

What are trans-isomers?

Answer 19

A trans-isomer is a type of geometric isomer where two identical or similar groups are positioned on opposite sides of a double bond or a ring structure. It is the opposite of the cis-isomer, where the groups are on the same side.

Key Points:
1. Opposite Side Arrangement: In a trans-isomer, the identical or similar groups are positioned on opposite sides of the double bond or ring.
2. Common in Alkenes and Cyclic Compounds: Trans-isomerism is often seen in alkenes (molecules with a carbon-carbon double bond) and certain cyclic compounds.
3. Physical Properties: Trans-isomers typically have different physical and chemical properties from their cis-isomers (where the groups are on the same side), such as differences in melting points, boiling points, and polarity.

Example:
- Trans-but-2-ene: In this molecule, the two methyl groups (CH₃) are on opposite sides of the double bond between the two carbon atoms.
- Trans-2-butene has a more symmetrical structure than the cis isomer, often leading to different properties, such as lower polarity.

The term trans helps distinguish the relative positioning of groups and plays a crucial role in understanding the molecule’s properties and reactivity.

Question 20

What type of reactions can alkenes undergo to produce an alkane? Provide a catalyst for this reaction.

Answer 20

They can undergo electrophilic addition with hydrogen to produce alkanes. The C=C bond opens up and forms single bonds to teach of the hydrogen atoms. This reaction requires a nickel catalyst.

CH₂=CHCH₃ + H₂ → CH₃CH₂CH₃
(Propene + Hydrogen → Propane)

Question 21

Define halogenaoalakanes

Answer 21

Organic compounds with single carbon bonds only and halogen functional groups. Alkenes undergo addition reactions with halogens to form di-substituted halogenoalkanes and with hydrogen halides to form mono-substituted halogenoalkanes. The mechanism for this reaction is given on the following page of these notes.

Question 22

Define di-substitution

Answer 22

Di-substitution refers to a chemical situation where two substituent groups (atoms or groups of atoms) replace two hydrogen atoms or other groups on a molecule. This term is often used when describing the substitution of two atoms or groups on a benzene ring or another molecule.

Question 23

Define mono-substitued haogenoalkanes

Answer 23

Mono-substituted halogenoalkanes are organic compounds where a single halogen atom (fluorine, chlorine, bromine, or iodine) has replaced one of the hydrogen atoms in an alkane molecule.

Question 24

What are alcohols, what type of functional group do they have, and what reaction should alkenes go through to form alochols?
What catalyst is needed for this reaction?
Give a reaction example.

Answer 24

Alcohols are organic compounds with a hydroxyl functional group.
Alkenes undergo addition reactions with steam to form alcohols. This reaction requires an acid catalyst, such as phosphoric acid.

CH₂=CH₂ + H₂O → CH₃CH₂OH

Question 25

Define diols

Answer 25

A diol is an organic compound that contains two hydroxyl groups (–OH) attached to two different carbon atoms in its structure.

Key Points:
Two Hydroxyl Groups: The key feature of a diol is the presence of two –OH (hydroxyl) groups, which are alcohol functional groups.
Position of Hydroxyl Groups: The hydroxyl groups can be located in different positions within the molecule, such as adjacent carbon atoms (vicinal diols) or separated by one or more carbon atoms (geminal diols).

Types of Diols:
Vicinal Diols: The two hydroxyl groups are attached to adjacent carbon atoms.
Geminal Diols: The two hydroxyl groups are attached to the same carbon atom, though these are rare and often unstable.

Example:
Ethylene glycol (C₂H₆O₂): A common diol where two hydroxyl groups are attached to adjacent carbon atoms. It is commonly used in antifreeze and as a coolant.
Butane-2,3-diol (C₄H₁₀O₂): A diol where the hydroxyl groups are attached to carbon atoms 2 and 3 in a butane chain.

Diols are important in organic chemistry because they can participate in various chemical reactions, such as forming polymers or undergoing oxidation reactions.

Question 26

What type of reaction must alkenes go through to form diols?
And what should it react with to oxidise the double bond?
Should you carry out this reaction in cold or hot solutions, must the solution be reacted with a diluted or not diluted and an acidified or not acidified solution?
Give a reaction example

Answer 26

Diols can be formed from alkenes through an oxidation reaction. The double bond is oxidised by acidified potassium manganate (VII) (KMnO4). The manganate ions must be cold, dilute and acidified.

CH₂=CH₂ + H₂O + (O) → CH₂(OH)CH₂(OH)