4.1.3 - Alkenes

• 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