Module 4.1.3 - Alkenes
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What are alkenes?
Unsaturated hydrocarbons (have at least one multiple bond between carbon atoms)
Why are alkenes unsaturated?
In alkanes, there is at least one double bond
What is the general formula for alkenes?
CnH2n
What bond is in a C=C bond?
Made up of 1 sigma bond and 1 pi bond (which restricts its rotation)
How is a sigma bond formed in an alkene?
Directly between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms
How is a pi bond formed in an alkene?
The double, sideways overlap of p-orbitals ABOVE AND BELOW the bonding C atoms
What shape and bond angle is around the carbons in C=C?
Trigonal planar, 120 degrees 3 bonding regions of electrons which repel each other equally to get as far apart as possibly
How do you name alkenes?
• Find the longest C chain that includes the alkene group & name the stem • Number parent chain from end to lowest number possible for alkene group • If more than 1 alkene group, use prefix (di-,tri-..) and add an ‘a’ at the end of stem of alkene • Name and number any alkene groups, with multiple list in alphabetical order by name • Cyclic alkenes have the prefix ‘-cyclo’
What is stereoisomerism?
Compounds with the same structural formula but with a different arrangement of the atoms in space.
What does E/Z stand for?
Entgegen/Zusammen
What is the criteria for E/Z stereoisomerism? (2)
• Must have a carbon-carbon double bond, so the compound cannot rotate • EACH carbon atom of the C=C must have two different groups attached to it
How do you draw E/Z stereoisomerism?
Draw the molecule with its trigonal planar shape and attached groups are displayed in structural formula, has to condense only to the C=C
How do you find priority in alkene stereoisomers?
Compare the atoms that are attached on either side of the C=C and look at their atomic number (top and bottom) Higher atomic number = Higher priority
How do you identify a Z-stereoisomer?
The highest priority groups are on the same side of the double bond (either both highest priority groups are top, top OR bottom, bottom)
How do you identify an E-stereoisomer?
Highest priority groups are on opposite sides of the C=C (one highest priority group on bottom, one on top)
What is the criteria for cis-trans stereoisomerism? (3)
• must have a carbon-carbon double bond because it cannot rotate • Each carbon on the C=C must have two different groups attached • Two groups on the C=C must be identical
How do you identify a cis-stereoisomer?
The two same groups from each C atom are adjacent to the C=C
How do you identify a trans-stereoisomer?
Two same groups from each C atom are opposite the C=C bond (across from each other)
Why are alkenes more reactive than alkanes?
Alkenes have C=C bond, which is their functional group
What type of reactions do alkenes undergo?
Addition reactions - A group is added across the C=C double bond
Why does the pi bond break when an alkene reacts?
It has relatively low bond enthalpy
Why is the atom economy 100% for alkenes?
Since addition, only one product is formed so no waste product
What are the different reactions for alkenes? (5)
• Bromination • Chlorination • Hydrogenation • Hydration • Hydrogen halide
What is the test for identifying alkenes?
Bromine water (bromination) as it goes from orange to colourless