organic (paper 2) Flashcards
1
Q
What are alkanes?
A
Alkanes are saturated hydrocarbons, meaning they only contain carbon (C) and hydrogen (H) atoms
2
Q
What is the general formula for alkanes?
A
CₙH₂ₙ₊₂
3
Q
What is the structure of alkanes?
A
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.
4
Q
What is the boiling point trend for alkanes?
A
As the number of carbon atoms in an alkane increases, the boiling point increases. This is due to increased van der Waals forces
5
Q
What is the state of alkanes at room temperature?
A
alkanes with 1- 4 carbon atoms are gases at room temperature Alkanes with 5-17 carbon atoms are liquids
Alkanes with more than 18 carbon atoms are solids
6
Q
What is the difference between straight-chain and branched-chain alkanes?
A
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.
7
Q
What is the combustion of alkanes?
A
The combustion of alkanes is an exothermic reaction
CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O
The reaction releases energy in the form of heat and light.
8
Q
What is incomplete combustion?
A
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.
9
Q
What are the main sources of alkanes?
A
Alkanes are primarily obtained from natural gas and crude oil. They are extracted through fractional distillation
10
Q
What is the reactivity of alkanes?
A
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.
11
Q
What is a free radical substitution reaction?
A
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.
12
Q
What is the importance of alkanes in industry?
A
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.
13
Q
What is the process of cracking in alkanes?
A
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.
14
Q
What are isomers of alkanes?
A
Isomers of alkanes are compounds with the same molecular formula but different structural arrangements.
15
Q
What is the significance of the carbon-carbon bond in alkanes?
A
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.
16
Q
What are the environmental concerns with alkanes?
A
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.
17
Q
What are the properties of alkanes?
A
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.
18
Q
What are the two main types of isomers?
A
The two main types of isomers are:
Structural isomers (different bonding arrangement)
Stereoisomers (same bonding arrangement, but different spatial arrangement)
19
Q
What are structural isomers?
A
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.
20
Q
What are the types of structural isomers?
A
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)
21
Q
What is chain isomerism?
A
Chain isomerism occurs when the carbon chain in the molecule is arranged differently.
22
Q
What is functional group isomerism?
A
when the compounds have the same molecular formula but different functional groups.
23
Q
What is positional isomerism?
A
Positional isomerism occurs when the functional group is in a different position on the carbon chain.
24
Q
What are stereoisomers?
A
Stereoisomers have the same molecular and structural formula but differ in the spatial arrangement of atoms. They include cis-trans isomerism and optical isomerism.
25
What is cis-trans isomerism?
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.
26
What is the difference between cis and trans isomers in terms of physical properties?
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.
27
What are enantiomers?
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).
28
What is a chiral center (or stereocenter)?
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.
29
What is a racemic mixture?
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.
30
What is the relationship between cis-trans isomerism and alkenes?
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.
31
What is the impact of isomerism on chemical properties?
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.
32
What are alkenes?
Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C).
33
What is the general formula for alkenes?
CₙH₂ₙ
34
What is the structure of an alkene molecule?
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°.
35
Why are alkenes reactive?
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.
36
What are the physical properties of alkenes?
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.
37
What is the reaction of alkenes with halogens?
Alkenes react with halogens (e.g., chlorine or bromine) in an electrophilic addition reaction, forming dihalogenated compounds.
38
What is the reaction of alkenes with hydrogen (hydrogenation)?
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.
39
What is the reaction of alkenes with hydrogen halides?
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.
40
What is the reaction of alkenes with bromine water?
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.
41
What is polymerization of alkenes?
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.
42
What are the types of polymerization for alkenes?
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).
43
What is the effect of a double bond on the reactivity of alkenes?
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.
44
How does the presence of a double bond affect the shape of an alkene molecule?
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.
45
What are the environmental concerns related to alkenes?
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.
46
Why are alkenes more reactive than alkanes?
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.
47
What is the addition reaction of alkenes with water (hydration)?
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.
48
What is the equation for the hydration of ethene?
The hydration of ethene to form ethanol is:
C₂H₄ + H₂O → C₂H₅OH
49
What type of catalyst is used in the hydration of alkenes?
The hydration of alkenes typically uses an acid catalyst such as phosphoric acid (H₃PO₄) or sulfuric acid (H₂SO₄).
50
What is a carbocation?
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.
51
What is the order of stability of carbocations?
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.
52
What are polymers formed from alkenes?
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.
53
What are halogenoalkanes?
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.
54
What is the general formula for halogenoalkanes?
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.
55
What are the physical properties of halogenoalkanes?
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.
56
How does the size of the halogen atom affect the reactivity of halogenoalkanes?
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.
57
What is the mechanism of nucleophilic substitution in halogenoalkanes?
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.
58
What factors affect the mechanism of nucleophilic substitution?
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.
59
What is the role of the solvent in nucleophilic substitution reactions?
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.
60
What is the difference between SN1 and SN2 mechanisms?
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.
61
What is the effect of the structure of the halogenoalkane on the nucleophilic substitution rate?
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.
62
What is the reaction of halogenoalkanes with aqueous sodium hydroxide (OH⁻)?
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).
63
What are some common uses of halogenoalkanes?
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.
64
What are the environmental concerns related to halogenoalkanes?
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.
65
How can halogenoalkanes be synthesized?
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.
66
What is the role of halogenoalkanes in the formation of ozone-depleting substances?
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.
67
What is the general formula for alcohols?
The general formula for alcohols is CₙH₂ₙ₊₁OH, where n is the number of carbon atoms in the molecule.
68
How are alcohols classified?
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.
69
What is the reaction of alcohols with sodium?
Alcohols react with sodium to produce an alkoxide ion and hydrogen gas. For example: 2Na + 2ROH → 2RONa + H₂
70
How do alcohols react with acids?
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).
71
What is the process of fermentation?
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
72
How can alcohols be oxidized?
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.
73
What is the oxidation of ethanol?
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).
74
What is the oxidation of a secondary alcohol?
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).
75
What is the reaction of alcohols with sodium dichromate (K₂Cr₂O₇)?
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.
76
What is the mechanism of alcohol oxidation?
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.
77
How are alcohols dehydrated to form alkenes?
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.
78
What is the process of esterification?
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.
79
What is an ester functional group?
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.
80
How do alcohols behave as weak acids?
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⁺
81
What is the solubility of alcohols in water?
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.
82
What are the advantages of using ethanol as a biofuel?
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.
83
How are alcohols prepared by reduction?
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).
84
What are the key features of an alcohol’s reactivity?
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.
85
What is the reaction between alcohols and a carboxylic acid to form an ester?
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
86
How do you test for a primary alcohol?
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.
87
What is the reaction of alcohols with carboxylic acids?
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.
88
What are some examples of alcohols used in industry?
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).
89
Describe fractional distillation.
- crude oil heated to boiling point of hydrocarbons
- evaporated vapours rise up fractionating tower
- lower temperatures higher up fractionating tower
- condense to liquid when they arrive at a tray lower than their boiling point
- short chain have lower boiling points
- condense higher up
- long chain have higher boiling points
- condense lower down
- bitumen left behind
90
Describe thermal cracking.
- C-C bonds undergo homolytic fission
- producing short chain hydrocarbons ending in a carbon free radical
- reactive free radicals form a variety of short chain molecules
- high proportion of alkenes produced
- H2 may be produced
91
Explain how you purify an organic liquid.
- solvent extraction in seperating funnel
- add organic liquid and aqueous sodium hydrogencarbonate solution to seperating funnel
- stopper and shake
- impurities are more soluble in aqueous solution so move into it
- leave to separate and periodically invert and open tap to release gas to prevent build up of pressure
- solution and liquid are immiscible so form separate layers
- open tap to run off layers into separate beakers
92
How can gas chromatography be improved?
- use mass spectrometer as the detector
- mass of compound is found as it exits the gas chromatography column
- allowing it to be identified more accurately
93
How do plasticisers work and what are they made of?
- esters
- between polymer molecules
- allow polymer molecules to slide over each other
- makes material flexible
94
How is biodiesel made?
- rapeseed oil + methanol
- NaOH catalyst
- methyl esters formed
- can be used as fuel in diesel vehicles
95
How is yield lost in aspirin production?
- crystals lost in filtering and washing
- some aspirin stays in solution
- side reactions occur
96
What are esters used for?
- animal fats and vegetable oils
- flavourings and perfumes
- plasticisers
- making soap
- making cleaning agents and solvents
97
What are hydrolysis reactions?
- additions of water/OH
98
What are the 2 ways to produce ethanol?
- hydration of ethene
- fermentation
99
What are the conditions for catalytic cracking?
- high temperature
- slight pressure
- zeolite catalyst
100
What is the observation in Fehling's test
fehlings tests for aldehydes
if the test is positive the blue solution will turn to a brick red precipate
101
What is the observation in Tollen's reagent test on aldehyde?
silver mirror forms
102
Why are CFCs harmful?
- C-Cl bond absorbs UV light
- undergoes homolytic fission to form Cl*
- Cl* catalyses break down of ozone into oxygen
103
Why is reflux used?
- so that vapours of reactants and intermediates that form
- condense and return to the reaction mixture
- so that they fully react
104
Why a the carboxylate ion stable?
- delocalisation
- negative charge spread over the COO group
105
Why is the filtrate suctioned in aspirin production?
- to separate aspirin crystals from solution containing impurities
106
equations to show the production of ehthanol to be carbon neutral
6CO2 + 6H2O → C6H12O6 + 6O2 photosynthesis
C6H12O6 → 2C2H5OH + 2CO2 fermentation
C2H5OH → 2CO2 + 3H2O when its burnt
107
what would increases the solubility of a substance
- temperature:
For most solid solutes (like sugar or salt in water), increasing the temperature increases their solubility. This is because higher temperatures provide more energy to break the solute's intermolecular forces, allowing more of it to dissolve.
For gases, the solubility usually decreases with increasing temperature. This is because heat provides energy for gas molecules to escape the solution.
- pressure
- polarity:
Like dissolves like: Polar solutes dissolve better in polar solvents, and nonpolar solutes dissolve better in nonpolar solvents.
- size:
Smaller particles generally dissolve more easily than larger ones. This is because smaller particles have a greater surface area for the solvent to interact with, allowing faster dissolution.
