(A2) Alkenes and Alkanes Flashcards
What is an alkene?
A series of unsaturated hydrocarbons
What is an alkane?
A series of saturated hydrocarbons
Formula of Alkene
Cn H(2n)
Formula of Alkane
Cn H(2n+2)
What are saturated hydrocarbons?
Hydrocarbons that only contain single bonds between carbon atoms
What are unsaturated hydrocarbons?
Hydrocarbons that contain at least one carbon-carbon double bond
Type of Alkenes
- Straight Chain
- Branched Chain
- Cyclic Alkenes
Nomenclature for 1
Meth
Nomenclature for 2
Eth
Nomenclature for 3
Prop
Nomenclature for 4
But
What is cracking?
The breaking down of long chain hydrocarbons to smaller hydrocarbons
Types of Cracking
- Thermal Cracking
- Catalytic Cracking
What is catalytic cracking?
Using a catalyst to cause cracking
What is thermal cracking?
High temperature is used to cause cracking
Product of Cracking
1) Long Chain –> alkene + alkene
2) Long Chain –> alkene + alkene + H₂
What is combustion?
The burning of oxygen and fuel (alkane)
Types of Combustion
- Complete Combustion
- Incomplete Combustion
What is complete combustion?
There is large supply of oxygen
Formula of Complete Combustion
Alkane + Oxygen –> Carbon Dioxide + Water
CH₄ + O₂ –> CO₂ + 2H₂O
What is formed in complete combustion?
- Carbon Dioxide
- Water
What color is the flame in complete combustion?
Blue
What is incomplete combustion?
There is a limited supply of oxygen
Formula of Incomplete Combustion
Alkane + Oxygen –> Carbon Monoxide + Water
2CH₄ + 3O₂ –> 2CO + 4H₂O
What is formed in incomplete combustion?
- Carbon Monoxide
- Soot
- Water
- Carbon
What color is the flame in incomplete combustion?
Orange
Why is incomplete combustion undesirable?
It releases less energy and produces carbon monoxide(poisonous) unlike complete combustion which releases the maximum amount of energy from the fuel being reacted
What is Benzene?
A planar molecule where all the atoms lie in one plane which contains a ring of six carbon atoms that form a perfectly regular hexagon
What is the formula for Benzene?
C₆H₆
Which type of bond is shorter, double or single?
Double Bond
What bond angle is Benzene?
120°
Order of Bond Strength (alkenes, alkanes and benzene)
Strongest I
I Alkenes
I Benzene
I Alkanes
Weakest I
What is the bond length in Benzene?
All the bonds in benzene are the same length (0.14 nm)
What is the bond length of single and double bonds?
0.147 nm for single
0.135 nm for double
What is hybridisation?
When two orbitals fuse together
What are sigma bonds?
A covalent bond that occurs from the end to end overlap between orbitals
Symbol for Sigma Bonds
σ
What are pi Bonds?
A covalent bond that occurs from the side to side overlap between orbitals
Symbol for pi Bonds
𝛑
What orbital overlaps does a sigma bond have?
P-P (end to end)
S-P (end to end)
S-S (end to end)
What orbital overlaps does a pi bond have?
P-P (side by side)
How many σ and 𝛑 does a single bond have?
1 σ bond and 0 𝛑 bonds
How many σ and 𝛑 does a double bond have?
1 σ bond and 1 𝛑 bonds
How many σ and 𝛑 does a triple bond have?
1 σ bond and 2 𝛑 bonds
What is an alkyne?
A triple bond
What is stronger a sigma or a pi bond?
A sigma bond
Differences between sigma and pi bond
- A sigma bond has lower bond enthalpy than a pi bond
- A sigma bind has electron density between bonding atoms whilst a pi bond has electron density above and below bonding atoms
What is the bond angle for sp¹?
180°
What is the bond angle for sp²?
120°
What is the bond angle for sp³?
109.5°
What is a free radical?
A species with an unpaired electron
What is fission?
Splitting
What is fusion?
Combining
Types of free radical fission
- Heterolytic Fission
- Homolytic Fission
What is heterolytic fission?
One bonding atom receives both electrons from the bonding pair, ions are formed
What is homolytic fission?
Each bonding atom receives one electron from the bonding pair forming two radicals
What are the reagents in free radical substitution?
A halogen and an alkane
What is condition is needed for free radical substitution?
UV light
Stages of Free Radical Substitution (FRS)
- Initiation
- Propagation
- Termination
What happens during the initiation step?
Radicals are formed
How many equations does the initiation step have?
One
How many equations does the propagation step have?
Two
What happens during the termination step?
Radicals join to form products
How many equations does the termination step have?
At least one (Usually three)
Why is FRS not a good method?
Because biproducts are formed
Why are free radicals reactive?
Because they are trying to pair up their unpaired electron
What does inter mean?
In between the molecules
What does intra mean?
Inside the molecules
What are the types of isomers
- Structural Isomers
- Stereoisomers
What are structural isomers?
Compounds that have the same molecular formula but different structural arrangement
What are stereoisomers?
Compounds that have the same structural formula but different arrangement in space
What is a cis isomer?
Atoms that are on the same side of the double bond
What is the alternative name for the cis isomer?
Z isomer
What is a trans isomer?
Atoms that are on the opposite sides of the double bond
What is the alternative name for the trans isomer?
E isomer
What determines the priority of the isomers
The higher the atomic number of the atom attached to the carbon the higher its priority
e.g. Br>Cl>H
What is a carbocation?
A carbon with only three bonds and a positive charge
What is a “R” Group?
A chain of carbon atoms (CH₃)
What is a primary carbocation?
One of the three groups is an “R” group
H
H C CH₃
+
What is a secondary carbocation?
Two of the three groups is an “R” group
CH₃
H C CH₃
+
What is a tertiary carbocation?
All of the three groups is an “R” group
CH₃
H₃C C CH₃
+
Order of Stability of Carbocations
Highest I
I Tertiary
I Secondary
I Primary
Lowest I
Why is tertiary the most stable out of all the carbocations?
Because there are more “R” groups which adds more electrons
What are most reactions of alkenes?
Additional reaction
What are electrophiles?
A species that can accept a pair of electrons
They are either positive ions or polar molecules with partial charges
What are electrophiles attracted to?
high electron density carbon double bonds
What happens in electrophilic addition if the alkene is symmetrical?
The two carbocation are identical which leads to one product
What happens in electrophilic addition if the alkene is asymmetrical?
There are two different carbocations which leads to two possible products
What is the major product?
The more stable carbocation during electrophilic addition
What is the minor product?
The less stable carbocation during electrophilic addition
What needs to be added in an electrophilic addition diagram?
- Direction Arrow
- Dipole
- Charges
- Lone Pair