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chemistry > 4.3 Alkenes > Flashcards

4.3 Alkenes Flashcards

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1
Q

What are alkenes ?

A

Alkenes are unsaturated hydrocarbons, meaning they have at least one carbon = carbon double bond.

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2
Q

What is the general formula of alkenes ?

A

Cn H2n

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3
Q

What type of alkene obeys the general formula ?

A

Branched alkenes do obey the formula, but cyclic alkenes and alkenes with more than one double bond do not.

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4
Q

What is the general formula for alicyclic alkenes ?

A

Cn H2n-2

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5
Q

What are sigma bonds ?

A

They are single bonds and also are the first bond in the double bond of an alkene.

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6
Q

How are sigma bonds formed ?

A

Formed from end-on overlap of atomic orbitals (Eg., s-orbital and p-orbital).
3 of 4 electrons in carbon are used in 3 sigma bond and the 4th electron remains in the p-orbital.

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7
Q

What is a pi bond ?

A

Sideways overlap of p-orbitals and make up second bond in the double bond. The electron density is concentrated above and below the line going the carbon atoms.

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8
Q

Which bond is weaker (pi or sigma) ?

A

Pi bonds are weaker than sigma bonds.

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9
Q

What is the shape of alkenes if carbon forms 4 sigma bonds ?

A

If carbon atoms form 4 bonding pairs, the bonds angle is 109.5 in a tetrahedral arrangement.

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10
Q

What is the shape of bond around carbon atoms of double bond in an alkene ?

A

Trigonal planar shape with a bond angle of 120 as there are 3 regions of electron density where pi bond is present.

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11
Q

What is an isomer ?

A

Molecule with same molecular formula but different structural formula.

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12
Q

What are stereoisomers ?

A

They are molecules with the same molecular formula and same structural formula but different 3D arrangement of atoms in a space.

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13
Q

What is the criteria for stereoisomers ?

A

-Must have C=C double bond.
-There must be different atoms attached to each carbon atom of the double bond.

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14
Q

What is E/Z isomerism ?

A

Arises as there is restricted rotation around the double bond. The relative position of attached atoms are fixed. Rotation of the molecule around the double bond would results in breaking sideways overlap of p-orbitals and breaking the pi bond and therefore breaks the double bond.

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15
Q

How can we identify which is E or Z isomers ?

A

The atoms attached to each carbon of the double bond are given priority. The higher the atomic number, the higher the priority.

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16
Q

What is a z isomer ?

A

Atoms with higher priority, attached to the carbons of double bond, are on the SAME SIDE.

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17
Q

What is an E isomer ?

A

Atoms with higher priority, attached to carbons of double bond, are on opposite sides.

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18
Q

What is the criteria for cis-trans isomers ?

A

-C=C double bond is present.
-Each carbon atom of double bond is bonded to 2 different atoms.
-One of attached groups on each carbon atom of double bond must be the same.

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19
Q

What is a cis isomer ?

A

When the same group is on the same side (top or bottom of double bond), then the isomer is classed as cis.

20
Q

What is a trans isomer ?

A

When the same group is on different sides of the double bond, then it is classed as trans isomer.

21
Q

What type of bond breaks during reaction of alkenes ?

A

Pi bond breaks as it is weaker so breaks more easily and forms bond to molecule being added.

22
Q

What is hydrogenation of alkenes and wbhat conditions are required ?

A

Is when an alkene is mixed with hydrogen and passes over a nickel catalyst as 423K to form an alkane. Hydrogen is added across the double bond.

23
Q

What is halogenation of alkenes ?

A

Alkenes undergo a rapid addition reaction with halogens at room temperature. Halides are added across the double bond.

24
Q

What is addition of hydrogen halides with alkenes ?

A

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. Hydrogen and halide from across the double bond.

25
Q

What is hydration reaction of alkenes ?

A

Alkenes react with steam (H2O) to form alcohols in the presence of phosphoric acid catalyst.

26
Q

What is an electrophile ?

A

An atom or group of atoms that are attracted to and electron rich centre and accepts an electron pair to form a covalent bond.

27
Q

What is heterolytic fission ?

A

The breaking of a covalent bond with the electron pair going to one atom so forming a cation and an anion.

28
Q

What is a carbocation ?

A

A species containing a positively charged carbon atom (with only 6 electrons ).

29
Q

What is electrophilic addition ?

A

-Less electronegative atom is attracted to alkene.
-electron pair in Pi bond is attracted to positive atom.
-Bond forms with atom and alkene.
-Electron pair moves from bond to anion.
-Anion reacts with carbocation to form product.

30
Q

What is a primary, secondary and tertiary carbocation ?

A

Primary = 0 or 1 alkyl groups attached to carbon of double bond.
Secondary = 2 alkyl groups …
Tertiary = 3 alkyl groups …..

31
Q

Why is the tertiary carbocation the most stable ?

A

The alkyl groups push the electron density toward the positive charge of the carbocation. This spreads the positive charge over the alkyl group and increases stability.

32
Q

What is Markownikoffs rule ?

A

When an unsymmetrical reagent (HBr) reacts with an unsymmetrical alkene, the major product is formed via the more stable carbocation. (The carbon with most hydrogens get the least electronegative atom).

33
Q

According to Markownikoffs rule, what happens if molecule bonding to unsymmetrical alkene is symmetrical ?

A

If atoms are the same then atom closest to alkene gets an induced dipole and binds to alkene. There is no major and minor products as molecule binding to alkene is the same, so would form the same product.

34
Q

What is a polymer ?

A

A large molecule of repeating units of monomers.

35
Q

What is addition polymerisation of alkenes?

A

Unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains, wit no double bonds. This occurs by the pi bond breaking.

36
Q

What is a repeat unit ?

A

Specific arrangement of atoms in a polymer molecule that repeats over and over.

37
Q

Do addition polymers have a high molecular formula ?

A

Yes

38
Q

Why are alkene based polymer an environmental concern ?

A

Environmental concern as they are mostly non-biodegradable.

39
Q

How does recycling polymers reduce the environmental risks ?

A

Conserves fossil fuels and reduces amount of waste going to landfill.

40
Q

How do we recycle polymers?

A

They have to be sorted by type and then melted and used for new products.

41
Q

How do we recycle polymer that are difficult to recycle ?

A

They are burnt to produce heat and generate fuel.

42
Q

What is feedstock recycling ?

A

Chemical and thermal processes to reclaim monomers. Products resemble those of crude oilcan be used to form new polymers.

43
Q

What are bioplastics ?

A

Produced from plant starch, cellulose, plant oils and proteins. They are renewable and sustainable as it conserves finite resources.

44
Q

What are biodegradable polymers ?

A

They can be broken down over time by microorganisms into H20, CO2 and other organic compounds.

45
Q

What are compostable polymers ?

A

Degrade and leave no toxic/ harmful residues.

46
Q

What are photodegradable polymers ?

A

They are oil-based polymers that contain bonds that are weakened by absorbing light to start degradation.
Also, light absorbing additives are used.

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