organic I (p2)

Question 1

What are alkanes?

Answer 1

Alkanes are saturated hydrocarbons, meaning they only contain carbon (C) and hydrogen (H) atoms, with single bonds between the carbon atoms. They follow the general formula CₙH₂ₙ₊₂.

Question 2

What is the general formula for alkanes?

Answer 2

The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms in the molecule.

Question 3

What is the structure of alkanes?

Answer 3

Alkanes have a tetrahedral structure around each carbon atom, with bond angles of approximately 109.5°. The carbon atoms are connected by single covalent bonds (sigma bonds), and each carbon atom forms four bonds.

Question 4

What is the boiling point trend for alkanes?

Answer 4

As the number of carbon atoms in an alkane increases, the boiling point increases. This is due to increased van der Waals forces (induced dipole–dipole interactions) between larger molecules.

Question 5

What is the state of alkanes at room temperature?

Answer 5

Alkanes with 1-4 carbon atoms are gases at room temperature (e.g., methane, ethane). Alkanes with 5-17 carbon atoms are liquids (e.g., pentane, hexane). Alkanes with more than 18 carbon atoms are typically solids (e.g., paraffin wax).

Question 6

What is the difference between straight-chain and branched-chain alkanes?

Answer 6

Straight-chain alkanes have all carbon atoms in a continuous chain, while branched-chain alkanes have one or more branches of carbon atoms attached to the main chain.

Question 7

What is the combustion of alkanes?

Answer 7

The combustion of alkanes is an exothermic reaction where alkanes react with oxygen to produce carbon dioxide and water:
CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O
The reaction releases energy in the form of heat and light.

Question 8

What is incomplete combustion?

Answer 8

Incomplete combustion occurs when there is insufficient oxygen, leading to the formation of carbon monoxide (CO) and/or soot (carbon particles) instead of carbon dioxide. It is less efficient and produces harmful gases.

Question 9

What are the main sources of alkanes?

Answer 9

Alkanes are primarily obtained from natural gas and crude oil. They are extracted through fractional distillation and refined to produce fuels and other products.

Question 10

What is the reactivity of alkanes?

Answer 10

Alkanes are relatively unreactive due to the strong sigma bonds between carbon atoms and the stability of the molecule. They undergo combustion and substitution reactions but are generally resistant to addition reactions.

Question 11

What is a free radical substitution reaction?

Answer 11

Free radical substitution is a reaction where a hydrogen atom in an alkane is replaced by a halogen atom (such as chlorine or bromine). This reaction typically occurs in the presence of ultraviolet (UV) light and involves the formation of free radicals.

Question 12

What is the importance of alkanes in industry?

Answer 12

Alkanes are important as fuels (e.g., methane, propane, butane), solvents, and feedstocks for the production of chemicals like plastics, lubricants, and synthetic materials. They are also key in petrochemical industries.

Question 13

What is the process of cracking in alkanes?

Answer 13

Cracking is a process used to break down large alkane molecules (typically from crude oil) into smaller, more useful molecules, such as alkenes and shorter alkanes. This process can be catalytic or thermal.

Question 14

What are isomers of alkanes?

Answer 14

Isomers of alkanes are compounds with the same molecular formula but different structural arrangements. These can include straight-chain and branched-chain isomers. For example, butane has two isomers: n-butane and isobutane.

Question 15

What is the significance of the carbon-carbon bond in alkanes?

Answer 15

The carbon-carbon single bond (sigma bond) in alkanes is strong and stable, making alkanes relatively unreactive. However, this bond can be broken under conditions of high temperature or with the presence of free radicals, such as during combustion or substitution reactions.

Question 16

What are the environmental concerns with alkanes?

Answer 16

The burning of alkanes, especially in fuels, contributes to air pollution and greenhouse gas emissions, such as carbon dioxide (CO₂). Incomplete combustion can also produce carbon monoxide (CO), a toxic gas.

Question 17

What are the properties of alkanes?

Answer 17

Alkanes are non-polar molecules, making them insoluble in water. They are also less dense than water, and their boiling points increase with the length of the carbon chain due to stronger van der Waals forces.

Question 18

What is the significance of methane (CH₄)?

Answer 18

Methane is the simplest alkane, a major component of natural gas, and an important fuel. It is also a potent greenhouse gas when released into the atmosphere.

Question 19

What are isomers?

Answer 19

Isomers are compounds that have the same molecular formula but different structural arrangements of atoms. They can have different chemical and physical properties.

Question 20

What are the two main types of isomers?

Answer 20

The two main types of isomers are:

Structural isomers (different bonding arrangement)
Stereoisomers (same bonding arrangement, but different spatial arrangement)

Question 21

What are structural isomers?

Answer 21

Structural isomers are compounds with the same molecular formula but different structural formulas. They differ in the connectivity of atoms or the arrangement of atoms in the molecule.

Question 22

What are the types of structural isomers?

Answer 22

The main types of structural isomers are:

Chain isomerism (different carbon chain arrangements)
Functional group isomerism (different functional groups)
Positional isomerism (functional group in different positions)

Question 23

What is chain isomerism?

Answer 23

Chain isomerism occurs when the carbon chain in the molecule is arranged differently. For example, butane has two chain isomers: n-butane (a straight chain) and isobutane (a branched chain).

Question 24

What is functional group isomerism?

Answer 24

Functional group isomerism occurs when the compounds have the same molecular formula but different functional groups. For example, alcohols and ethers can be functional group isomers.

Question 25

What is positional isomerism?

Answer 25

Positional isomerism occurs when the functional group is in a different position on the carbon chain. For example, 1-bromobutane and 2-bromobutane are positional isomers, as the bromine atom is attached to different carbons in the chain.

Question 26

What are stereoisomers?

Answer 26

Stereoisomers have the same molecular and structural formula but differ in the spatial arrangement of atoms. They include cis-trans isomerism and optical isomerism.

Question 27

What is cis-trans isomerism?

Answer 27

Cis-trans isomerism occurs when there is restricted rotation around a double bond or ring structure, leading to different spatial arrangements of substituents.

Cis: Same substituents are on the same side of the double bond.
Trans: Substituents are on opposite sides of the double bond.

Question 28

What is the difference between cis and trans isomers in terms of physical properties?

Answer 28

Cis and trans isomers often have different physical properties. For example, cis-isomers tend to have higher boiling points due to stronger intermolecular forces, while trans-isomers tend to be more symmetrical and thus have lower boiling points.

Question 29

What are enantiomers?

Answer 29

Enantiomers are a pair of molecules that are mirror images of each other but cannot be superimposed. They often have identical physical properties except for the way they rotate plane-polarized light (optical activity).

Question 30

What is a chiral center (or stereocenter)?

Answer 30

A chiral center is a carbon atom that is bonded to four different atoms or groups. This makes the molecule non-superimposable on its mirror image, resulting in optical isomerism.

Question 31

What is a racemic mixture?

Answer 31

A racemic mixture is a 1:1 mixture of two enantiomers. Because the optical activity of the enantiomers cancels each other out, a racemic mixture is optically inactive.

Question 32

What is the relationship between cis-trans isomerism and alkenes?

Answer 32

Alkenes can exhibit cis-trans isomerism when each carbon in the double bond is bonded to two different substituents. This creates a scenario where the substituents can be on the same side (cis) or opposite sides (trans) of the double bond.

Question 33

Can alkanes exhibit stereoisomerism?

Answer 33

Alkanes generally do not exhibit stereoisomerism because they do not contain double bonds or chiral centers. However, branched alkanes can have structural isomers.

Question 34

What is the impact of isomerism on chemical properties?

Answer 34

Isomerism can affect the reactivity and physical properties (such as boiling point, solubility, and polarity) of compounds. For example, the presence of different functional groups in structural isomers leads to different chemical reactivity.

Question 35

What are alkenes?

Answer 35

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C). They follow the general formula CₙH₂ₙ for compounds with one double bond.

Question 36

What is the general formula for alkenes?

Answer 36

The general formula for alkenes is CₙH₂ₙ, where n is the number of carbon atoms in the molecule.

Question 37

What is the structure of an alkene molecule?

Answer 37

Alkenes have a carbon-carbon double bond (C=C), which consists of one sigma (σ) bond and one pi (π) bond. The carbon atoms in the double bond are sp² hybridized, and the bond angles around these carbon atoms are approximately 120°.

Question 38

Why are alkenes reactive?

Answer 38

Alkenes are reactive due to the presence of the pi (π) bond in the double bond, which is weaker than a sigma bond and more easily broken, making alkenes more prone to addition reactions.

Question 39

What are the physical properties of alkenes?

Answer 39

Alkenes are non-polar molecules, which makes them insoluble in water but soluble in organic solvents. They have similar boiling points to alkanes with the same number of carbon atoms, and their boiling points increase with the size of the molecule.

Question 40

What is the reaction of alkenes with halogens?

Answer 40

Alkenes react with halogens (e.g., chlorine or bromine) in an electrophilic addition reaction, forming dihalogenated compounds. For example, ethene reacts with bromine to form 1,2-dibromoethane.

Question 41

What is the reaction of alkenes with hydrogen (hydrogenation)?

Answer 41

Alkenes undergo hydrogenation in the presence of a catalyst (usually nickel or platinum) to form alkanes. The double bond is broken, and hydrogen atoms are added to each carbon, turning the unsaturated alkene into a saturated alkane.

Question 42

What is the reaction of alkenes with hydrogen halides?

Answer 42

Alkenes react with hydrogen halides (e.g., HCl, HBr) in an electrophilic addition reaction to form alkyl halides. For example, ethene reacts with HCl to form chloroethane.

Question 43

What is the reaction of alkenes with bromine water?

Answer 43

Alkenes decolorize bromine water due to an electrophilic addition reaction. The bromine (Br₂) adds across the double bond, turning the orange-brown solution colorless and forming a vicinal dibromide.

Question 44

What is polymerization of alkenes?

Answer 44

Polymerization is the process by which small alkene monomers (like ethene) undergo an addition reaction to form long-chain polymers (e.g., polyethylene). This reaction is initiated by heat, pressure, or a catalyst, causing the double bonds to open and link the monomers together.

Question 45

What are the types of polymerization for alkenes?

Answer 45

The two main types of polymerization are:

Addition polymerization: Monomers with a double bond add together to form a polymer.
Condensation polymerization: Monomers with two functional groups react, releasing a small molecule (often water).

Question 46

What is the effect of a double bond on the reactivity of alkenes?

Answer 46

The double bond in alkenes makes them more reactive than alkanes. The electron-rich pi bond is more likely to undergo reactions such as electrophilic addition and polymerization.

Question 47

How does the presence of a double bond affect the shape of an alkene molecule?

Answer 47

The presence of a double bond in an alkene leads to a planar shape around the double-bonded carbons, with bond angles of approximately 120° due to sp² hybridization of the carbon atoms.

Question 48

What are the environmental concerns related to alkenes?

Answer 48

The production and use of alkenes, especially in plastic manufacturing (e.g., polyethylene), contribute to plastic waste. Additionally, some alkenes, like ethene, are involved in the formation of ozone in the atmosphere, which can be harmful in large quantities.

Question 49

Why are alkenes more reactive than alkanes?

Answer 49

Alkenes are more reactive than alkanes because the pi bond in the carbon-carbon double bond is weaker than the sigma bond. The pi bond is more exposed to attack by electrophiles, making alkenes susceptible to addition reactions.

Question 50

hat is the addition reaction of alkenes with water (hydration)?

Answer 50

Hydration is the addition of water (H₂O) to an alkene in the presence of an acid catalyst (e.g., H₃PO₄ or H₂SO₄). The alkene reacts with water, breaking the double bond and adding a hydroxyl group (OH) and a hydrogen atom to the carbon atoms, forming an alcohol.

Question 51

What is the equation for the hydration of ethene?

Answer 51

The hydration of ethene to form ethanol is:
C₂H₄ + H₂O → C₂H₅OH

Question 52

What type of catalyst is used in the hydration of alkenes?

Answer 52

The hydration of alkenes typically uses an acid catalyst such as phosphoric acid (H₃PO₄) or sulfuric acid (H₂SO₄).

Question 53

What is a carbocation?

Answer 53

A carbocation is a positively charged species where a carbon atom has only six electrons in its valence shell, making it highly reactive. Carbocations are intermediates in many electrophilic addition reactions involving alkenes.

Question 54

What is the order of stability of carbocations?

Answer 54

The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon. The order of stability is: tertiary > secondary > primary > methyl.

Question 55

What are polymers formed from alkenes?

Answer 55

Alkenes can polymerize to form long-chain polymers, such as polyethylene, polypropylene, and polystyrene, through addition polymerization. This process involves the breaking of the carbon-carbon double bond to form a continuous chain of repeating monomer units.

Question 56

What are halogenoalkanes?

Answer 56

Halogenoalkanes, also known as alkyl halides, are organic compounds in which one or more halogen atoms (fluorine, chlorine, bromine, iodine) are bonded to a carbon atom of an alkane chain.

Question 57

What is the general formula for halogenoalkanes?

Answer 57

The general formula for halogenoalkanes is CₙH₂ₙ₊₁X, where X is a halogen (Cl, Br, I, or F), and n is the number of carbon atoms in the molecule.

Question 58

What are the physical properties of halogenoalkanes?

Answer 58

Halogenoalkanes generally have higher boiling points than alkanes of similar size due to the polarity of the C-X bond. The boiling point increases with the size of the halogen atom. Halogenoalkanes are often more dense than water and are insoluble in water but soluble in organic solvents.

Question 59

How does the size of the halogen atom affect the reactivity of halogenoalkanes?

Answer 59

The reactivity of halogenoalkanes increases as the halogen atom becomes larger. This is because the C-I bond is weaker (due to lower bond dissociation energy) compared to C-Cl or C-Br, making it easier to break and initiate nucleophilic substitution or elimination reactions.

Question 60

What is the mechanism of nucleophilic substitution in halogenoalkanes?

Answer 60

Nucleophilic substitution involves the replacement of the halogen atom (X) with a nucleophile (e.g., hydroxide ion, OH⁻). This reaction can proceed via two mechanisms:

SN1 (Unimolecular nucleophilic substitution): Involves a two-step mechanism with the formation of a carbocation intermediate.
SN2 (Bimolecular nucleophilic substitution): Involves a one-step mechanism where the nucleophile attacks the carbon atom simultaneously as the halogen leaves.

Question 61

What factors affect the mechanism of nucleophilic substitution?

Answer 61

The mechanism of nucleophilic substitution depends on:

The structure of the halogenoalkane: Primary halogenoalkanes favor SN2; tertiary halogenoalkanes favor SN1.
The leaving group: Halogens make good leaving groups, with iodine being the best, followed by bromine, chlorine, and fluorine.
The nucleophile: A strong nucleophile (e.g., OH⁻ or CN⁻) favors SN2, while weaker nucleophiles favor SN1.

Question 62

What is the role of the solvent in nucleophilic substitution reactions?

Answer 62

The solvent can influence the mechanism:

Polar aprotic solvents (e.g., acetone, DMSO) favor the SN2 mechanism by not solvating the nucleophile, allowing it to attack the carbon more effectively.
Polar protic solvents (e.g., water, alcohols) favor the SN1 mechanism as they stabilize the carbocation intermediate and the leaving group.

Question 63

What is the difference between SN1 and SN2 mechanisms?

Answer 63

SN1 Mechanism: Occurs in two steps. The halogen leaves first to form a carbocation, followed by nucleophilic attack. Tends to occur with tertiary halogenoalkanes and weaker nucleophiles.
SN2 Mechanism: Occurs in one step, where the nucleophile attacks the carbon from the opposite side of the leaving group, displacing it. This mechanism favors primary halogenoalkanes and strong nucleophiles.

Question 64

What is the effect of the structure of the halogenoalkane on the nucleophilic substitution rate?

Answer 64

Primary halogenoalkanes favor SN2 substitution and react faster.
Secondary halogenoalkanes can undergo both SN1 and SN2, with SN2 being more common in polar aprotic solvents.
Tertiary halogenoalkanes favor SN1 substitution due to the stability of the carbocation intermediate.

Question 65

What is the reaction of halogenoalkanes with aqueous sodium hydroxide (OH⁻)?

Answer 65

Halogenoalkanes react with aqueous sodium hydroxide in a nucleophilic substitution reaction, producing alcohols. For example, bromomethane (CH₃Br) reacts with sodium hydroxide to form methanol (CH₃OH).

Question 66

What are some common uses of halogenoalkanes?

Answer 66

Halogenoalkanes are used as solvents, refrigerants, intermediates in organic synthesis, and in the production of pharmaceuticals. For example, chloroform (CHCl₃) was used as an anesthetic, and CFCs were used in refrigeration and air conditioning.

Question 67

What are the environmental concerns related to halogenoalkanes?

Answer 67

Some halogenoalkanes, particularly chlorofluorocarbons (CFCs), have been found to deplete the ozone layer, leading to increased ultraviolet radiation reaching the Earth’s surface. This can cause skin cancer and damage to ecosystems.

Question 68

How can halogenoalkanes be synthesized?

Answer 68

Halogenoalkanes can be synthesized by:

Radical substitution: Halogenation of alkanes in the presence of UV light.
Electrophilic substitution: Halogenation of aromatic compounds.
Nucleophilic substitution: Reaction of alcohols with hydrogen halides.

Question 69

What is the role of halogenoalkanes in the formation of ozone-depleting substances?

Answer 69

Halogenoalkanes, specifically CFCs, release chlorine atoms when they break down in the stratosphere, which catalytically destroy ozone molecules (O₃). This leads to the thinning of the ozone layer, contributing to global warming and increased UV radiation.

Question 70

What is the general formula for alcohols?

Answer 70

The general formula for alcohols is CₙH₂ₙ₊₁OH, where n is the number of carbon atoms in the molecule.

Question 71

How are alcohols classified?

Answer 71

Alcohols can be classified based on the number of alkyl groups attached to the carbon bearing the hydroxyl group (-OH):

Primary alcohols: The hydroxyl group is attached to a carbon atom that is bonded to one alkyl group.
Secondary alcohols: The hydroxyl group is attached to a carbon atom bonded to two alkyl groups.
Tertiary alcohols: The hydroxyl group is attached to a carbon atom bonded to three alkyl groups.

Question 72

What is the reaction of alcohols with sodium?

Answer 72

Alcohols react with sodium to produce an alkoxide ion and hydrogen gas. For example: 2Na + 2ROH → 2RONa + H₂

Question 73

How do alcohols react with acids?

Answer 73

Alcohols react with acids, such as sulfuric acid, to form alkyl hydrogen sulfate or ester (when reacting with carboxylic acids). For example, ethanol reacts with acetic acid to form ethyl acetate (an ester).

Question 74

What is the process of fermentation?

Answer 74

Fermentation is the biological conversion of glucose (C₆H₁₂O₆) into ethanol (C₂H₅OH) and carbon dioxide (CO₂) by yeast. The reaction is as follows: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Question 75

How can alcohols be oxidized?

Answer 75

Alcohols can be oxidized to form aldehydes, ketones, or carboxylic acids, depending on the type of alcohol and the reaction conditions. Primary alcohols are oxidized to aldehydes and further to carboxylic acids. Secondary alcohols are oxidized to ketones, while tertiary alcohols do not undergo oxidation under typical conditions.

Question 76

What is the oxidation of ethanol?

Answer 76

Ethanol (a primary alcohol) can be oxidized to acetaldehyde (an aldehyde) by an oxidizing agent such as potassium dichromate (K₂Cr₂O₇) under acidic conditions. Further oxidation can convert acetaldehyde to acetic acid (a carboxylic acid).

Question 77

What is the oxidation of a secondary alcohol?

Answer 77

Secondary alcohols are oxidized to ketones. For example, propan-2-ol (a secondary alcohol) is oxidized to acetone (a ketone) by an oxidizing agent like potassium dichromate (K₂Cr₂O₇).

Question 78

What is the reaction of alcohols with sodium dichromate (K₂Cr₂O₇)?

Answer 78

Alcohols react with sodium dichromate in the presence of sulfuric acid to undergo oxidation. Primary alcohols are oxidized to aldehydes and then carboxylic acids, while secondary alcohols are oxidized to ketones. Tertiary alcohols do not react under these conditions.

Question 79

What is the mechanism of alcohol oxidation?

Answer 79

The oxidation mechanism involves the removal of two hydrogen atoms from the alcohol molecule (one from the hydroxyl group and one from the carbon atom). This is typically achieved by an oxidizing agent like potassium dichromate (K₂Cr₂O₇) in an acidic solution.

Question 80

How are alcohols dehydrated to form alkenes?

Answer 80

Alcohols can undergo dehydration (removal of water) to form alkenes when heated with a strong acid like concentrated sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄). This is an example of an elimination reaction.

Question 81

What is the process of esterification?

Answer 81

Esterification is the reaction between an alcohol and a carboxylic acid (or its derivative) in the presence of an acid catalyst (e.g., concentrated sulfuric acid) to form an ester and water. For example, ethanol reacts with acetic acid to form ethyl acetate and water.

Question 82

What is an ester functional group?

Answer 82

An ester functional group has the general formula RCOOR’, where R and R’ are alkyl groups. Esters are formed from the reaction between an alcohol and a carboxylic acid.

Question 83

How do alcohols behave as weak acids?

Answer 83

Alcohols are weak acids because they can donate a proton (H⁺) from the hydroxyl group (-OH). However, they are much weaker acids compared to carboxylic acids. The dissociation of alcohols is represented as:
ROH ⇌ RO⁻ + H⁺

Question 84

What is the solubility of alcohols in water?

Answer 84

Alcohols are generally soluble in water due to hydrogen bonding between the hydroxyl group of the alcohol and water molecules. The solubility decreases as the carbon chain length increases because the nonpolar alkyl group becomes more dominant.

Question 85

What are the advantages of using ethanol as a biofuel?

Answer 85

Ethanol is used as a biofuel because it is renewable, produced from plants (such as sugarcane or maize), and burns more cleanly than fossil fuels, emitting less carbon dioxide and particulate matter. It is commonly blended with gasoline for use in internal combustion engines.

Question 86

How are alcohols prepared by reduction?

Answer 86

Alcohols can be prepared by the reduction of aldehydes and ketones. For example:

Aldehydes are reduced to primary alcohols.
Ketones are reduced to secondary alcohols. Reduction typically requires a reducing agent such as sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄).

Question 87

What are the key features of an alcohol’s reactivity?

Answer 87

Alcohols are relatively reactive due to the polar nature of the hydroxyl group (-OH). They can:

Undergo oxidation to form aldehydes, ketones, or carboxylic acids.
Participate in nucleophilic substitution and esterification reactions.
React with sodium to form alkoxides.

Question 88

What is the reaction between alcohols and a carboxylic acid to form an ester?

Answer 88

The reaction between an alcohol and a carboxylic acid in the presence of an acid catalyst (often sulfuric acid, H₂SO₄) is known as esterification. This produces an ester and water. For example, ethanol reacts with acetic acid to form ethyl acetate and water: C₂H₅OH + CH₃COOH → C₂H₅COOCH₃ + H₂O

Question 89

How do you test for a primary alcohol?

Answer 89

Primary alcohols can be tested by oxidation with an oxidizing agent like potassium dichromate (K₂Cr₂O₇). The reaction causes the orange solution to turn green as the alcohol is oxidized to an aldehyde or further to a carboxylic acid.

Question 90

What is the reaction of alcohols with carboxylic acids?

Answer 90

Alcohols react with carboxylic acids in the presence of an acid catalyst (e.g., concentrated sulfuric acid) to form esters. For example, ethanol reacts with acetic acid to form ethyl acetate and water.

Question 91

What are some examples of alcohols used in industry?

Answer 91

Ethanol (used as a solvent, in alcoholic beverages, and as a biofuel).
Methanol (used as a solvent and in the production of formaldehyde).
Isopropanol (used as a disinfectant and solvent).