chapter 13 alkenes Flashcards
(53 cards)
what are unsaturated hydrocarbons?
hydrocarbons containing one or more carbon carbon double bonds
what is the general formula of alkenes?
CnH2n
what is a pi bond?
a bond formed by the sideways overlap of two p-orbitals, containing two electrons and with the electron density concentrated above and below the line joining the nuclei of the bonded atoms
what does a pi bond do to alkenes?
it locks the two carbon atoms in position and prevents them from rotating around the double bond. this makes geometry of alkenes different to geometry of alkanes.
what are the bond angles possible around C in an alkene?
109.5/tetrahedral if in a C-C single bond and 120/trigonal planar if in a C=C double bond
what is stereoisomerism?
compounds with the same structural formula but with a different arrangement of atoms in space.
what are the two types of stereoisomerism?
E/Z isomerism and optical isomerism. E/Z only occurs in compounds with a C=C double bond but optical can occur in alkanes with no functional groups too
why does stereoisomerism occur?
rotation around the double bond is restricted and therefore the groups attached to each C in the double bond are fixed relative to each other.
what two factors does a molecule need to satisfy to have E/Z isomerism?
•a C=C double bond
•different groups attached to each carbon in the double bond
Can but-1-ene and but-2-ene form E/Z isomers?
but-1-ene cannot because on C1 there are two hydrogens attached to it. but-2-ene can because it has a methyl group and a hydrogen attached to each C in the double bond
what is cis/trans isomerism?
a special type of E/Z isomerism in which there are two non hydrogen groups and two hydrogen atoms around the around the C=C double bond. the cis isomer (Z) has hydrogen atoms adjacent to each other and the trans isomer (E) has hydrogen atoms opposite each other.
what is E/Z isomerism?
type of stereoisomerism in which different groups attached to each carbon of a C=C double bond may be arranged differently in space because of its restricted rotation of the C=C bond.
what do the cahn-ingold-prelog rules say about stereoisomerism?
the atoms attached to each carbon in the double bond are given a priority based on atomic number (higher atomic number means higher priority).
are alkenes or alkanes more reactive? why?
alkenes are more reactive. this is because of the presence of the pi bond (pi electrons are more exposed than sigma electrons so a pi bond breaks breaks more easily).
what is a C=C double bond made up of?
a pi bond and a sigma bond.
what are the four common examples of alkene addition reactions?
•hydrogen in the presence of a nickel catalyst
•halogens
•hydrogen halides
•steam in the presence of an acid catalyst
each of these involves the addition of a small molecule causing the pi bond to break
what is hydrogenation of an alkene?
hydrogen is added across the double bond breaking the double bond and forming the alkane companion to the alkene
how do we test for unsaturation?
if (orange) bromine water is added drop wise to a sample of an alkene then the bromine adds across the double bond meaning the orange colour disappears indicating the presence of a double bond. if the same test is carried out with a saturated compound then there is no colour change
how does the reaction of an alkene with a hydrogen halide happen?
alkenes react with gaseous hydrogen halides to form haloalkanes. if the alkene is a gas then the gases are mixed if the alkene is liquid then the hydrogen halide is bubbled through it. alkenes also react with concentrated hydrochloric or hydrobromic acid which are solutions of the hydrogen halides in water.
what is formed from the hydration of alkenes?
alcohols are formed when an alkene reacts with steam in the presence of a H3PO4 catalyst
what is halogenation of alkenes?
alkenes undergo rapid addition reaction with chlorine or bromine at room temperature forming a dihaloalkane
what is electrophilic addition?
an addition reaction in which the first step is attack by an electrophile on a region of high electron density. it is a mechanism for how alkenes form saturated compounds. for alkenes specifically the double bond represents high electron density because of pi electrons
what is an electrophile?
an atom or group of atoms that is attracted to an electron rich centre where it accepts a pair of electrons
Describe the mechanism for the reaction between but-2-ene and H-Br.
•Br is more electronegative than H so H-Br is polar and contains a partially positive H and a partially positive Br.
•The electron pair in the pi bond is attracted to the partially positive H atom causing double bond to break.
•A bond forms between the H atoms of H-Br molecule and a C atom that was part of the double bond
•The H-Br bond breaks by heterolytic fission with the e- pair going to the bromine atom
•a Br- ion and a carbocation form
•Br- reacts with carbocation to form addition product
