2.8 Alkenes Flashcards
What is the key feature of an alkene?
Isolated C=C
How many components does a alkene double bond have?
Two
1. Sigma framework of molecule (orbitals on the different atoms facing one another) - sp2 from each overlap
2. Pi component - overlap of 2 non-hybridised p orbitals, one on each C (high electron density above and below flat molecule)
What is the polarity of a C=C
Non-polar (not free to rotate)
Sigma bonds can rotate without bonds breaking
What are E/Z isomers?
E = high priority functional groups on opposite sides
Z = high priority functional groups on the same side
What is the order of priority of functional groups?
C2H5 > CH3 > H & I > Br > Cl > F > C > H
What’s the strongest force seen in molecules with just carbon carbon, and carbon hydrogen bonds ONLY?
Van der vaals forces
Size of molecule controls strength of molecule
What differences are there in chemical reactivity between alkananes and alkenes?
- alkenes more reactive than alkanes
- alkenes tend to undergo addition reactions (particularly electrophicic addition in c=c)
In infrared spectroscopy, what are the stretch ranges for alkene C=C and C-H?
C=C 1680 - 1640 cm-¹
C-H 3100 - 3000 cm-1
Not the best method for identifying alkenes. Non-polar so weak signals
What are the ¹³C and ¹H nmr spectroscopy ranges for alkenes?
¹³C = 100 - 170 ppm
¹H = ~5-7 ppm (characteristic coupling)
Which hydrogens show a small coupling constant in alkenes?
2 hydrogens on the same carbon - don’t easily interact with one another (germinal coupling 0-3Hz)
Which hydrogens show a medium coupling constant in alkenes?
2 hydrogens which are 3 bonds apart on same side of double bond - cis coupling (6-14Hz)
Which hydrogens show the largest coupling constant in alkenes?
When there’s 2 hydrogens on opposite side of double bond - very good bond arrangement - trans coupling (11-18Hz)
What is the 1st method of alkenes synthesis?
By elimination of H-X from hydrogen alkanes (eg. HI, HBr)
What reagents and conditions are needed for the 1st method of synthesising alkenes?
Reagent: Base eg. Sodium or potassium hydroxide
Conditions: reflux in alcoholic solution
What is a 1 step elimination reaction involving 2 molecules described as?
E2 elimination
Molecularity of 2
Stereospecific mechanism - 1 to 1
What is the 2nd type of alkene synthesis?
Again a dehalogenation reaction (removal of a halogen) BUT of vicinal dibromides
Molecule of Br2 removed (each on differnt carbon, 1 and 2)
What are the reagents needed for the 2nd method of alkene synthesis?
- NaI
- Zn/ desolved in HOAc - acetic acid (redox reaction)
Iodide acts as a nucleophile and attacks Br
What is the 3rd method of alkene synthesis?
Dehydration of alcohols - elimination of water molecule
Made easy with acids
Primary alcohol - Strong acid required
Teriary alcohol - weaker acid. Lower Ea. Less heat required
What reagents and conditions are required to synthesise alkenes from alcohols (3rd method)?
Reagent: catalytic acid eg. Sulfuric or phosphoric
Conditions: Reflux (can depend on alcohols structure)
What is the difference in temperature and reagents required when synthesising alkenes from primary and tertiary alcohols?
Primary: conc. H2SO4 + 180°C
Secondary: 20% H2SO4 + 85°C
Which metabolic pathways eliminate water to make alkenes?
- Krebs cycle (form cis aconite acid / fumaric acid)
- Saturated hydrocarbon (chemically unreactive) > remove hydrogen as a radical > creates a cis double bond in fatty acids > alkene is formed
Print off slide with table and learn the types of alkene addition reactions
How many steps are involved in the electrophic addition of alkenes?
Two
- Arrow from double bond (breaking it) to the slightly positive terminal of the electrophile. H of H-Cl
- Arrow from bond in the H-Cl (for example) to the halogen. Indicating breaking
- H, electrophile is added to the previous alkene
- Carbocation reacts with Cl- nucleophile
What allows for 2 products of electrphilic addition reaction?
Asymmetric alkene or asymmetric electrophile
Major and minor products
Discovered by Markovnikov
What is Markovnikov’s rule?
H-X/water added to the double bond in a way that leads to the more stable carbocation. Major product arises from this.
This is the side of alkene with higher number of hydrogens attached to it (forms major product)
What does adding more alkyl groups to a carbocation do to stability?
Increase
Teriary are most stable > secondary > primary are unstable (methyl carbocation)
How is an alkene reduced?
By addition of hydrogen
H2 gas used
Heterogeneous catalyst required - usually transition metal such as Pt, Pd or Ni
What does a transition metal catalyst do in alkene reduction?
It absorbs the starting material and hydrogen onto its surface.
Pulls some electron density out of the hydrogen bond and makes it more reactive
In what form do enzymes biologically move around H2?
H- (comes from cofactor eg. NAD(P)H )
H+ (comes from any acidic recidues) from enzyme or water
What recation forms diols (OH on either side) from an alkene? Reagents?
Dihyroxylation reaction
Reagent: osmium tetroxide (OsO4) / Cold Alkaline KMnO4
What would hot KMnO4 do to an alkene?
Split it in half and form ketones/carboxylic acids
What is used to obtain an aldehyde from an alkene?
Ozone (O3)
How is the opposite distribution of major and minor products, of the addition of H-Br in Ionic reaction achieved?
Through radical addition
Must have an organic peroxide to begin with - Br adds before the H
What are the 3 requirements for radical polymer formation?
High pressure
High temperature
Initiator
Can be difficult to control due to radical high reactivity
