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Chemistry Unit 2 > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

What does the C=C bond consist of?

A

A sigma (σ) bond and a pi (π) bond

The sigma bond is formed by head-on overlap of orbitals, while the pi bond is formed by sideways overlap of p-orbitals.

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2
Q

How is a sigma bond formed?

A

By the head-on overlap of orbitals

Each carbon atom donates one electron to the electron pair in the σ-bond.

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3
Q

How is a pi bond formed?

A

By sideways overlap of p orbitals

Each carbon atom donates one electron to the electron pair in the π-bond.

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4
Q

What is the shape around the double bond in alkenes?

A

Trigonal planar

This shape is due to three areas of electron density around each carbon atom in the double bond.

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5
Q

What is the bond angle in a trigonal planar shape?

A

120°

This angle is characteristic of the geometry around the double bond in alkenes.

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6
Q

What is the bond length of a C=C bond compared to a C-C bond?

A

The C=C bond has a shorter bond length than the C-C bond

Bond length is the distance between the nuclei of two covalently bonded atoms.

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7
Q

Why are alkenes more reactive than alkanes?

A

Due to the C=C double bond

The presence of the double bond increases reactivity compared to single-bonded alkanes.

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8
Q

What is the bond enthalpy of a C-C single bond?

A

347 kJ mol-1

This value represents the energy required to break the bond.

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9
Q

What is the bond enthalpy of a C=C double bond?

A

612 kJ mol-1

This value indicates that the double bond is stronger than the single bond.

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10
Q

Fill in the blank: A pi bond is a covalent bond formed by _______.

A

sideways overlap of p orbitals

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11
Q

Fill in the blank: A sigma bond is a covalent bond formed by _______.

A

linear overlap of atomic orbitals

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12
Q

What is the relationship between the strength of the pi bond and the sigma bond in alkenes?

A

The pi bond is slightly weaker than the sigma bond, making it easier to break.

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13
Q

Why are alkenes more reactive than alkanes?

A

Alkenes are more reactive due to the presence of the C=C bond, which has high electron density.

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14
Q

What are electrophiles?

A

Species attracted to areas of high electron density, such as the C=C bond in alkenes.

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15
Q

Give two examples of electrophiles that react with alkenes.

A
  • Halogens
  • Hydrogen halides
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16
Q

What type of reaction occurs when electrophiles react with alkenes?

A

Electrophilic addition reaction.

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17
Q

Fill in the blank: The reaction of C2H4 + HBr results in _______.

A

C2H5Br

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18
Q

Fill in the blank: The reaction of C2H4 + Br2 results in _______.

A

1,2-dibromoethane

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19
Q

What is formed when C2H4 reacts with H2?

A

Ethane.

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20
Q

Describe the first step of the electrophilic addition mechanism of HBr to ethene.

A

HBr acts as an electrophile and attacks the pi bond, forming a carbocation intermediate.

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21
Q

What happens to the H-Br bond during the first step of the electrophilic addition?

A

The H-Br bond breaks, releasing a bromide ion.

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22
Q

What is a carbocation intermediate?

A

A positively charged species formed during the electrophilic addition reaction.

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23
Q

What occurs in the second step of the electrophilic addition mechanism?

A

A lone pair of electrons from the bromide ion forms a covalent bond with the carbocation.

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24
Q
A
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25
26
What do curly arrows represent in organic mechanisms?
The movement of an electron pair ## Footnote Curly arrows are used to illustrate how electrons are transferred during chemical reactions.
27
Define an electrophile.
An ion or molecule that attacks regions of high electron density ## Footnote Electrophiles are often positively charged or neutral species that can accept an electron pair.
28
How do halogens react with alkenes?
More slowly than other electrophiles ## Footnote The non-polar halogen-halogen bond requires a specific interaction with the alkene's double bond to initiate a reaction.
29
What happens when a bromine molecule approaches an alkene's double bond?
The bonding pair of electrons is repelled, giving the nearest halogen atom a partial charge ## Footnote This partial charge allows the bromine to act as an electrophile.
30
What is the product formed when ethene reacts with bromine?
1,2-dibromoethane ## Footnote This reaction involves the addition of bromine across the double bond of ethene.
31
What is a diagnostic test for the alkene functional group?
The use of bromine solution ## Footnote The orange/yellow/brown color of bromine solution decolorizes in the presence of a C=C double bond.
32
What color change occurs when bromine solution reacts with an alkene?
Decolorization ## Footnote This indicates the presence of a double bond in the alkene.
33
What can be said about the detection of chlorine's color change in testing for alkenes?
It is too difficult to detect due to its pale green/colorless nature ## Footnote Unlike bromine, chlorine water does not provide a clear visual indicator for the presence of alkenes.
34
What type of products are formed from unsymmetrical alkenes like prop-1-ene?
A mixture of products in different amounts ## Footnote The addition of electrophiles to unsymmetrical alkenes can lead to varying product distributions based on stability.
35
In the reaction of prop-1-ene with HBr, which product is considered major?
2-bromopropane ## Footnote This product is formed preferentially due to the stability of the carbocation intermediate.
36
In the reaction of prop-1-ene with HBr, which product is considered minor?
1-bromopropane ## Footnote This product results from a less stable carbocation intermediate compared to the major product.
37
38
39
What is the effect of alkyl groups on carbocation stability?
Alkyl groups exert a positive inductive effect, increasing the stability of a carbocation as the number of alkyl groups attached increases. ## Footnote Alkyl groups push electron density towards the positively charged carbon atom.
40
Define a primary carbocation.
A primary carbocation has one alkyl group attached to the positively charged carbon (C+). ## Footnote It is the least stable type of carbocation.
41
Define a secondary carbocation.
A secondary carbocation has two alkyl groups attached to the positively charged carbon (C+) and is more stable than a primary carbocation. ## Footnote Stability increases with more alkyl groups.
42
Define a tertiary carbocation.
A tertiary carbocation has three alkyl groups attached to the positively charged carbon (C+) and is more stable than both primary and secondary carbocations. ## Footnote Tertiary carbocations are the most stable due to the maximum inductive effect.
43
What is the electrophilic addition mechanism of HBr to propene?
The mechanism involves the formation of two possible carbocation intermediates: a more stable carbocation and a less stable carbocation. ## Footnote The stability of the carbocation determines the major and minor products formed.
44
What is the major product when HBr reacts with propene?
2-bromopropane is the major product formed from the more stable carbocation. ## Footnote The more stable carbocation favors the formation of this product.
45
What is the minor product when HBr reacts with propene?
1-bromopropane is the minor product formed from the less stable carbocation. ## Footnote This product is less favored due to the stability of the intermediate.
46
Fill in the blank: A _______ carbocation has three alkyl groups attached to C+.
tertiary
47
True or False: A secondary carbocation is more stable than a tertiary carbocation.
False
48
Fill in the blank: The stability of a carbocation increases with the number of _______ groups attached.
alkyl
49
50
What is the first step in the addition of HBr to an alkene?
The hydrogen from the HBr adds to the double bond, resulting in the formation of a carbocation intermediate.
51
What type of carbocation is formed when HBr adds to the carbon with two hydrogen atoms?
Secondary carbocation.
52
What type of carbocation is formed when HBr adds to the carbon with one hydrogen and a methyl group?
Primary carbocation.
53
Which carbocation is more stable: primary or secondary?
Secondary carbocation.
54
Why is a secondary carbocation more stable than a primary carbocation?
The methyl group can help stabilize the positive charge.
55
What is the definition of a primary carbocation?
A carbocation which has one carbon atom directly bonded to the positively charged carbon.
56
What is the definition of a secondary carbocation?
A carbocation which has two carbon atoms directly bonded to the positively charged carbon.
57
What is the definition of a tertiary carbocation?
A carbocation which has three carbon atoms directly bonded to the positively charged carbon.
58
What does Markovnikov's Rule state about electrophilic additions?
All electrophilic additions proceed via carbocation intermediates, leading to major and minor products when both alkene and the added molecule are unsymmetrical.
59
What is hydrogenation in the context of alkenes?
The conversion of alkenes to alkanes in the presence of hydrogen gas and a nickel catalyst.
60
What is the industrial significance of hydrogenation?
It is important in the production of margarine and other soft spreads from vegetable oils.
61
What temperature is required for the hydrogenation of ethene?
180 °C.
62
What are vegetable oils characterized by?
They are liquids containing long hydrocarbon chains, often with many double bonds, and are known as polyunsaturated.
63
What effect does partial hydrogenation have on unsaturated fats?
It can transform some Z (cis) double bonds into E (trans) double bonds.
64
What are E (trans) fats thought to be in terms of health?
Bad for health.
65
What is the main E fat found in hydrogenated vegetable oils?
E-octadec-9-enoic acid.
66
67
What are alkenes used for in industry?
Polymers ## Footnote Alkenes are industrially important for producing various polymers.
68
What is a monomer?
An individual alkene molecule that can join together to form a polymer ## Footnote Monomers are the building blocks of polymers.
69
What is polymerisation?
The joining together of many small molecules (monomers) to form a large molecule ## Footnote This process is essential for creating polymers.
70
What properties do polythene have?
Flexible, light, good insulator, resistant to water and acid ## Footnote Polythene is commonly used in plastic bags and bottles.
71
List some uses of polythene.
* Plastic bags * Bottles * Cable insulation ## Footnote These applications leverage its insulating properties.
72
What are the properties of polypropene?
Tough, easily moulded, resistant to water and acid ## Footnote Polypropene is used in crates, toys, and ropes.
73
What are some applications of PVC?
* Guttering * Window frames * Waterproof clothing ## Footnote PVC is known for being tough and rigid.
74
What are polythene, polypropene, and PVC commonly referred to?
Plastics ## Footnote These materials are chemically inert and produced relatively cheaply.
75
True or False: Plastics like polythene and PVC are reactive and easy to dispose of.
False ## Footnote Their lack of reactivity can cause problems when disposing of these plastics.
76
Fill in the blank: A polymer is a large molecule formed when _______ join together.
monomers ## Footnote Monomers are the small molecules that combine to form polymers.
77

Chemistry Unit 2 (11 decks)

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