4.1.3 - Alkenes

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Khads <3

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What is the molecular shape about each carbon atom in alkenes and why?
+ each carbon atom in a (C-C) double covalent bond contains three bonding regions and no lond pairs
+ molecular shape about each carbon atom must be trigonal planar
+ bond angle is 120 degrees about each carbon atom of double bond
What is the general formula of branched and unbranched alkenes?
CnH2n
How are alkenes structured generally?
+ alkenes are unsaturated molecules
+ contain at least one double carbon bond
+ cyclic alkenes and alkenes with more than one double bond do not have the same general formula of CnH2n
Why can free rotation occur around the C-C sigma bond in alkanes?
heaviest electron density in sigma bond exists along same plane line of C-C bond
How do sigma bonds form in alkenes?
+ Sigma bonds are simply single covalent bonds formed from a head-on overlap of orbitals
+ For each carbon atom, there are four outershell electrons
+ Three of the outershell electrons are used to form 3 sigma bonds
+ one electron from each carbon atom in double bond forms a sigma bond to other carbon atom of double bond
+ two electrons from each carbon atom in double bond forms two other sigma bonds to two other atoms (carbon or hydrogen)
How does pi bonding occur in alkenes?
+ fourth electron on each carbon atom of double bond not involved in sigma bonding
+ fourth electron occupies p orbital
+ pi bond formed by sideways overlap of p orbitals
+ pi bond creates area of electron density above and below plane of carbon atoms
Which bond is weaker, a pi bond or a sigma bond, and why?
+ pi bonds are weaker than sigma bonds
+ as sideways overlap of p orbitals has a smaller orbital overall than head-on overlap of orbitals forming a sigma bond
+ less energy required to break pi bonds
+ only pi bond breaks when alkenes react
What is the chemical test for alkenes?
+ bromine water looks orange-yellow
+ when alkenes added to bromine water, colour change from orange-yellow to colourless observed
+ highlights that alkenes contain unsaturated bonds that are reactive
What does the double bond do to the motion of the carbon atoms?
+ pi bond locks the carbon atoms in position
+ restricts carbon atoms from rotating around the double bond
+ makes geometry of alkenes different from alkanes
How are alkenes used in the natural world?
+ limonene is a cyclic alkene found in rinds of citrus fruits
+ naturally responsible for smell and flavour of oranges and lemons
+ limonen also used in perfumes and household cleaning products due to its fragrance
What is stereoisomerism?
have the same molecular and structural formula, but different arrangements of atoms in space
How do E-Z stereoisomers form?
result from different spatial arrangements of atoms or groups around double bond
What are the conditions for e-z isomerism?
+ restricted rotation of groups/ atoms around C=C double bonds
+ groups/ atoms bonded to each carbon of double bond are different
What is a Z isomer?
+ atom or group with highest atomic number on each carbon atom on the same side of double bond
What is an E isomer?
+ atom or group with highest atomic number on each carbon
+ opposite sides of double bond
What are the Cahn-Ingold-Prelog rules?
+ priority given to atoms or groups with highest atomic number
+ to decide E-Z steroisomerism
+ known as CIP rules
+ Br > Cl > F > H : highest to lowest priority
What is Cis-trans isomerism?
+ if atom or group attached to each carbon atom are same, then there is a special case of isomerism called cis-trans isomerism
What is a cis isomer?
when atom/ group attached to carbon atom are the same and on the same side of double bond
What is a trans isomer?
when atom/ group attached to carbon atom are same and on opposite sides of double bond
What are electrophiles?
electron pair acceptors
What is the chemical behaviour of electrophiles?
+ electron deficient
+ attracted to electron rich regions in other molecules
+ e.g. double bond in alkenes
What are three examples of electrophiles? 
+ hydrogen halides (HX)
+ halogen molecules (X2)
+ hydrogen molecules (H2)
Why are hydrogen halides electrophiles?
+ permanent dipoles exist in hydrogen halides
+ hydrogen atom in hydrogen halide has a slightly positive charge
+ halogen is more electronegative than the hydrogen atom
+ hydrogen atom is electron deficient
+ accepts electrons from electron rich regions such as alkene double bond
How are permanent dipoles induced in halogen molecules?
+ electron repulsion occurs between halogen molecule and electron rich alkene
+ permanent dipole created in halogen molecule to minimise repulsion
Why are halogen molecules electrophiles?
+ electron repulsion exists between electron rich double carbon bond of an alkene and electron cloud surrounding halogen molecule
+ due to high negative charges being in close proximity
+ to reduce repulsion, electron cloud in halogen molecule shifts away from electron rich double bond
+ induces a dipole in halogen molecule with slightly positive charge on halogen atom closest to alkene
+ slightly positive atom is able to accept electrons from double carbon bond
What is the general reaction of the electrophilic addition reactions of alkenes with hydrogen halides?
alkene + HX -> halogenoalkane
What is the general reaction mechanism of electrophilic addition reactions of alkenes with hydrogen halides?
alkene -> carbocation (intermediate) -> halogenoalkane
What is the bromination of alkenes?
+ chemical test for alkenes is an example of electrophilic addition
+ alkene added to bromine water causes colour change from orange-yellow to colourless
+ called a bromination reactions as bromine is added
What is the hydrogenation of alkenes are what are the conditions?
+ hydrogenation is a reaction where hydrogen is added across a double bond
+ turn alkenes into alkanes
+ high temperatures (150 degrees C)
+ Nickel catalyst, Ni
Do you get the same products from the addition reactions of different alkene stereoisomers?
+ Yes, product is saturated so there is no restricted rotation/ single C-C bonds to allow rotation in the final product
What is the hydration of alkenes?
+ steam added across a double bond
+ turn alkenes into alcohol
What are the conditions for the hydration of alkenes?
+ high temperature (300 degrees C)
+ high pressure (65 atm)
+ acid catalyst: concentration phosphoric acid (H3PO4) or concentrated sulfuric acid (H2SO4)
What is formed from the addition reactions with unsymmetrical alkene?
a minor and major product
When electrophiles react with unsymmetrical alkenes, how can different products be formed?
via different carbocations with varying stability
What is the order of increasing stability from primary, secondary to tertiary carbocations?
primary < secondary < tertiary
What is a primary carbocation?
A primary carbocation is a carbocation where the carbon atom bearing the positive charge is directly attached to one other carbon atom.
What is a secondary carbocation?
Carbocation with two alkyl groups attached.
What is a tertiary carbocation?
A carbocation with three alkyl groups attached to the positively charged carbon atom.
Which dipoles are formed in a water molecule when it reacts in the hydrogenation of alkenes?
slightly positive hydrogen bonded to slightly negative hydroxyl group
Hδ+ ----- OHδ-
What is the inductive effect?
+ the larger the number of alkyl groups attached to the positively charged carbon atom of the carbocation, the greater the inductive effect
+ increases stability of the carbocation
+ inductive effect - when electrons from each alkyl group are pushed towards the positively charged carbon atom
+ helps to reduce positive charge of carbon atom - makes carbocation more stable