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Alkenes Flashcards

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1
Q

What is a sigma bond

A

A covalent bond formed when atomic orbitals can overlap easily and directly

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2
Q

What type of bonding is found in single bonds

A

Sigma bonds

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3
Q

How’s does a sigma bond form, in O for example

A

Each oxygen atom has a half filled p orbital

P orbitals have lobes which stick out on each side of the nucleus

The love from each orbital containing the electrons will point towards each other directly and merge

which forms a sigma bond

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4
Q

Explain the rotation abilities of sigma bonds

A

Sigma bonds are free to rotate

Because rotating each nuclei has no effect on the bonding orbital

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5
Q

P shaped orbitals found around a nucleus (how many etc.)

A

3 p orbitals, px, py, pz

each directly 90 degrees apart

Sigma bond forms between the 2 that are directly facing, but the other 4 left over do not

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6
Q

Why are sigma bonds strong

A

As electrons in the bond are close to the nucleus of each atom

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7
Q

What is a pi bond

A

Covalent bonds formed from the sideways overlap of two p shaped atomic orbitals

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8
Q

Why are pi bonds weaker than sigma bonds

A

As electrons are further from the nucleus in a pi bond compared to a sigma bond

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9
Q

What does a pi bond create in terms of the nucleus

A

Two areas of electron density above and below the sigma bond

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10
Q

Explain the rotation of a pi bond

A

Cannot rotate freely because two areas of electrons density above and below the nucleus are formed which restricts the rotation

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11
Q

Whats a double bond, e.g in ethene

A

a sigma and pi bond

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12
Q

What is the bonding found in the carbon of an alkene double bond

A

Three sigma bonding pairs of electrons

Two pairs form sigma bonds with other atoms
One pair sigma bonds with another C

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13
Q

Describe and explain the shape and bonding angle of carbon

A

The three bonding pairs of electrons are in the plane of the molecule and repel each other

Molecule adopts a planar arrangement with bond angles of 120

The fourth bonding pair forms the double bond in combination with the C-C sigma bond

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14
Q

What are the two types of isomerism presented in alkenes

A

E/Z and cis/trans

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15
Q

When does cis/trans isomerism occur

A

When two of the atoms/ groups of atoms attached to each C atom of the C=C bond are the same

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16
Q

When does E/Z isomerism occur

A

When different atoms/ groups of atoms are attached to each carbon ion the C=C bond

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17
Q

Cis isomers

A

When two groups are on the same side of the double bond/ carbon ring (both above or both below)

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18
Q

Trans isomers

A

Two groups are on opposite sides of the double bond/ carbon ring (one above and one below C=C bond)

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19
Q

When does cis/trans become superseded by E/Z

A

When there are 3/4 different groups on the C=C

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20
Q

When can the cis/trans naming system be used with three atoms/ groups

A

-Two of the three groups are the same

These are on opposite sides of the bond

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21
Q

How is R1, r2, r3 and r4 places on the compound

A

R1 R2
C=C
R3 R4

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22
Q

How does CIP priority work

A

Look at the atomic number on the first atom attached to the carbon, r1

The higher the atomic number, the higher the priority

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23
Q

Determining E/Z isomerism

A
  1. Apply CIP Priority rules, see which of the two on the left and right have higher priority
  2. Deduce for E/Z
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24
Q

E isomerism

Study These Flashcards
A

Highest priority groups are on opposite sides of the C=C bond

25
Z isomerism
Highest priority groups are on the same side of the C=C bond, with above/below
26
For more complex cases of E/Z isomerism, what do you do
Look at the atoms attached closest to the C, then look at second and determine priority there
27
Are alkenes or alkanes more reactive and why
Alkenes are more reactive due to the presence of the pi bond in the C=C bond. The pi bond has a low bond enthalpy which means it's easier to break hence the more reactive nature of the alkenes.
28
What is hydrogenation, and what is it AKA
The reaction between an alkene and hydrogen. Can also be called reduction
29
What conditions are needed for a hydrogenation reaction to occur
A nickel catalyst A temperature of 200 degrees A pressure of 1000kPa
30
What is one application of a hydrogenation reaction
The production of margarine from vegetable oils. Vegetable oils are unsaturated and may be hydrogenated to make margarine. This has a higher mp due to stronger London forces.
31
What could controlling the conditions of hydrogenation mean for its applications (and how are the conditions controlled)
It is possible to restrict how many of the C=C bonds are broken, and produce partially hydrogenated vegetable oils These have the desired properties and textures for margarine manufacture
32
What is halogenation
The reaction between alkenes and halogens
33
What happens in a halogenation reaction (3)
-It is an example of an electrophilic addition, where an electrophile joins onto a double bond -The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms -The result of this is a dihaloalkane
34
Conditions required for a halogenation reaction
Occurs readily at room temperature
35
A halogenation reaction is the basis to which test
The test for unsaturation in molecules
36
What happens in the test for unsaturation
Bromine water, Br2, an orange or yellow solution is needed The unknown compound we are testing is shaken with the bromine water If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise
37
What is a hydrohalogenation reaction, and what is the product
The reaction between alkenes and hydrogen halides, to produce halogenoalkanes
38
What type of reaction is hydrohalogenation reaction and what condition is needed
Electrophillic addition reaction Reaction occurs quickly at room temperature
39
What is the order of the fastest reaction of hydrogen halides in a hydrohalogenation reaction and why
HI>HBr>HCl This is due to the increasing bond strength of the hydrogen-halogen bond, so the weakest bond reacts most easily
40
What is a hydration reaction
The addition of water to an alkene to form an alcohol. The water is added across the double bond
41
What are the conditions needed for a hydration reaction
Must be treated with steam at 300deg A pressure of 60 atmospheres needed A sulphuric acid or phosphoric acid catalyst
42
How is a hydration reaction processed
Via an intermediate, in which H+ and HSO4- ions are added across the double bond The intermediate is quickly hydrolysed by water, reforming the sulphuric acid
43
How is ethanol made
Via hydration. Reacting steam with ethene and a phosphoric catalyst, at a 300 deg temp and 60 atm pressure
44
Comment on the hydration reaction of ethene
Reversible reaction initially of a 5% percentage yield. Any unreacted alkenes are then recycled through and an overall yield of 90-95% can be attained.
45
Uses of hydration reaction in alkenes
Produces large quantities of ethanol, a widely used solvent and fuel
46
Benefits of a hydration reaction for producing ethanol, over fermentation
Process is much faster Process is higher yielding
47
Electrophile
Any positive ion/molecule which is attracted to an area of high electron density (e.g C=C bond)
48
Whats electrophilic addition
The addition of an electrophile to an alkene double bond, C=C
49
Why is the alkene double bond susceptible to attack by electrons
The pi bond sticks out a little bit and the double bond has a high electron density, meaning it's open to attack from electrophiles as they are electron deficit species.
50
What happens to the C=C bond in electrophilic addition
The C=C bond breaks and forms a single C-C bond Two new bonds are formed from each of the 2 carbon atoms
51
Why does the C=C bond have a high electron density
In the double bond there are both sigma and pi bonds. They both contain a pair of electrons each, giving the double bond four electrons which makes it high in density.
52
How do alkenes react
By electrophillic addition
53
Name 3 hydrogen halides and briefly explain how they are suitable for electrophilic addition
H-Cl, H-Br, H-I Halogen atoms are more electronegative than H, which means they attract electrons moreso than H. Dipoles form (positive on H, negative on the halides) This creates a polar molecule with a permanent dipole The H is there fore deficient in electrons, and is an electrophile
54
Electrophilic addition of a hydrogen halide, HBr (5) on ethene
1. The HBr molecule approaches the alkene 2. As H is an electrophile here, the positive partial charge on the H of the HBr bond is attracted to the high electron density of the double bond 3.The positive charge on H attracts a pair of electrons from the pi bond of the alkene and this electron pair moves towards the H atom. (curly arrow needed) 4. A H+ atom can only have one covalent bond. So, at the same time the pair of electrons in the H-Br covalent bond now moves to the Br atom. This is shown with another curly arrow.
55
Explain what type of fission is found in the electrophilic addition of hydrogen halides
Heterolytic fission, because the covalent bond is broken and both of the electrons in the bond are moved to the Br (one atom) and not evenly distributed
56
When are two products formed in electrophilic addition of hydrogen halides
When the alkene is unsymmetrical
57
Explain the formation of products in the electrophilic addition of the hydrogen halide, HBr (4) and what is final product
1. There is now a carbocation intermediate formed from the alkene, as the C is now positively charged as electrons have been lost from its pi bond 2. A negatively charged Br ion is formed. Its is negatively charged because both of the electrons that were in the covalent bond have now moved to the bromine and it has gained a pair. 3. The electron pair on the bromine ion are attracted to the positive carbocation, shown with a curly arrow 4. The electron pair now forms a covalent bond to the carbocation and the final product formed is bromoethane
58
What are 3 rules to follow when using curly arrows for electrophilic addition mechanisms
-Be double headed when showing the movement off a pair of electrons -Start from a lone pair of electrons or an area of high electron density -Move towards the delta positive electrophile or the positive charge of a carbocation
59

Chemistree (4 decks)

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