Chapter 13 Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons (containing at least one c-c double bond)
What is the general formula of non-alicyclic alkenes?
CnH2n
What type of bonds are present in C=C?
1 Sigma
1 Pi
How are Pi bonds formed?
Sigma bond must be formed first
Sideways overlap of p orbitals above and below the bonding carbon atoms
Restricts movement
Compare Sigma and Pi bonds
Pi Bonds have electron density above and below the bonded atoms, whilst Sigma bonds have electron density between the bonded atoms
Pi Bonds are a sideways overlap between two P orbitals, Sigma bonds are a head on overlap of two orbitals
The average bond enthalpy of a Pi bond is lower than that of a Sigma bond
Define Steroisomers
Compounds which have the same structural formula but a different arrangement of atoms in space
What is needed for a compound to have stereoisomers?
A Carbon to Carbon double bond (which restricts movement)
Each of the carbons across the C=C are bonded to two different groups
What is Cis/Trans isomerism? What is needed for this to occur?
A special type of E/Z stereoisomerism.
There must be a C=C
Each of the carbons across the C=C must be bonded to two different groups, with one of these having to be a hydrogen
What is the shape around the C=C and bond angle? Why?
Trigonal Planar. 120 degrees
There are 3 bonding regions and 0 lone pairs around the central carbon atoms.
The bonding region repel as far as possible, equally, with this shape and angle minimising repulsion the most.
What is CIP nomenclature?
The system for naming stereoisomers by assigning priority based on the first point of difference, with a higher priority given to atom with the higher atomic number
How are the higher priorities arranged for an E isomer?
Apart from each other. Looks like a Z, Chemistry is confusing.
Or A in apart is closer to E
How are the higher priorities arranged for a Z isomer?
On the same side, Together. Doesn’t look like a Z, so is Z. Chemistry is confusing.
Or T in together is closer to Z.
How are cis isomers arranged?
The hydrogens are on the same side
How are trans isomers arranged?
The hydrogens are on opposing sides
Why are Alkenes more reactive than Alkanes?
The electron density from the Pi bond is above and below the C=C, meaning the electrons are more exposed
Pi bonds are more readily broken, meaning Alkenes easily undergo addition reactions
What is the name of the addition reaction of synthesising an Alkane from an Alkene? What are the conditions and reagent?
Hydrogenation
Hydrogen as the reagent
Nickel as a catalyst
What is the name of the addition reaction of synthesising an dihaloalkane from an Alkene? What are the conditions and reagent?
Halogenation- reacting a halogen with an alkene
Reagent: Halogen
No extra conditions, normally room temperature
What is produced when a Hydrogen Halide reacts with an alkene? How is the experiment carried out? What is special about this?
Produces a haloalkane
If the alkene is not symmetrical, two products are produced, a major and minor
If both are gases, the gases are mixed to together
If the Alkene is a liquid, the Hydrogen Halide is bubbled in the liquid
What is the name of the addition reaction of synthesising an Alcohol from an Alkene? What are the conditions and reagent?
Hydration
Steam Reagent so high temperatures
Phosphoric Acid Catalyst
Also produces two products if unsymmetrical
Define an electrophile
An electron pair acceptor
(Species, usually slight or fully positive in charge, attracted to electron rich centre and accepts an electron pair)
What is the mechnanism name of the reaction of when a hydrogen halide reacts with an alkene?
Electrophilic Addition
Describe the reaction mechanism of a hydrogen halide reacting with an alkene
The electron pair in the Pi bond is attracted to the permanent dipole of the slightly positive hydrogen.
The hydrogen accepts an electron pair from the C=C, bonding with the alkene. The H-X bond break via heterolytic fission, producing a halide ion. The C=C breaks, forming a carbocation.
The halide is attracted to the positive charged carbocation, forming a covalent bond, forming a haloalkane.
Describe the reaction mechanism of a halogen reacting with an alkene
The electron pair in the Pi bond is attracted to the induced dipole of the slightly positive halogen.
The halogen accepts an electron pair from the C=C, bonding with the alkene. The X-X bond break via heterolytic fission, producing a halide ion. The C=C breaks, forming a carbocation.
The halide is attracted to the positive charged carbocation, forming a covalent bond, forming a haloalkane.
What is Markovnikov’s rule?
A rule that states the the major product of electrophilic addition will be produced via the more stable carbocation intermediate
Which type of carbocation is the most stable and why?
Tertiary carbocations (attached to 3 alkyl groups)
The alkyl groups donate electrons to the positive charge, helping to stabilise it
The more alkyl groups, the more the positive charge is stabilised, and the longer the intermediate exists and able to react to form the the major product
What conditions are needed for addition polymerisation?
High temperatures and pressure
What are 3 ways of disposing polymers?
Combustion/incineration to generate electricity (may need to deal with toxic byproducts e.g HCl)
Organic Feedstock Recycling to form new polymers
Cracking or converted into other useful organic products
Recycling
What is combustion of polymers? What are some advantages and disadvantages?
Incinerating polymers to produce heat
Ad: Used to heat turbines, produce electricity
Dis: Produces CO2, contributing to the enhanced green house effect
Can produce toxic gases like burning PVC to produce HCl gas- neutralise first, carbon capture
What is feedstock recycling? What are some advantages and disadvantages?
Reclaiming monomer to make new polymers
Ad: Polymers do not have to be sorted nor washed
Dis: Lots of energy needed for process, and large amounts of material are needed to the process to be cost efficient
What is the process of recycling PVC?
Grinding and reusing to manufacture new product
Use solvents to dissolve it, precipitate to produce high grade PVC
What are biodegradable polymers?
Polymers broken down by microorganisms to produce water, carbon dioxide…
Ad: Degrade with no toxic residues, more fuel efficient in the future
Dis: Expensive to produce
What are photodegradable polymers?
Polymers which contain bonds weakened by absorbing light/ adding light absorbing additives
Ad: Degrade so does not end up in land fill
Dis: Uncontrolled degrading when there is light- may mix with other plastics when recycling
