12. Alkenes and Alkynes Flashcards
(32 cards)
Alkene
-unsaturated hydrocarbon that contains C-C double bond
-bond angle: around 120°
-free rotation in C-C double bond does not normally take place
Alkyne
-unsaturated hydrocarbon that contains C-C triple bond
Ethylene
-simplest alkene
-trace amounts in nature; large artificial production
-natural ripening agent for fruits
-fruits can be treated with ethylene gas or ethephon (ethrel) which releases ethylene
-from thermal cracking of ethane
Thermal cracking
-saturated hydrocarbon is converted to unsaturated hydrocarbon plus H2
Cis-trans isomers
-isomers that have same connectivity of their atoms but different arrangement in space
-results from the presence of either a ring or C-C double bond
(i.e. cis-2-butene and trans-2-butene)
IUPAC naming
ALKENES
- Longest carbon chain that includes double bond. Indicate length of parent chain using prefix
- Number chain to give lowest number to carbon atoms of double bond. Designate the position of double bond by the number of its first carbon
- Branched alkenes are named in similar manner to alkanes
(i.e. 4-methyl-1-hexene; 2,3-diethyl-1-pentene)
IUPAC naming
ALKYNES
-similar lang sa alkenes except -yne suffix
-when both double and triple bonds are present, lowest number is given to the bond that comes first
-“–en” suffix follows the parent chain. Location of double bond is indicated before the parent name; location of triple bond is indicated between “–en” and “–yne”
(i.e. 3-penten-1-yne;
3,4-dipropyl-1,2-hexadien-5-yne;
1,4,4-trimethylcyclobutene)
Common names
-usually for low molecular weights
Ethene : Ethylene
Propene : Propylene
2-Methylpropene : Isobutylene
Ethyne : Acetylene
Propyne : Methylacetylene
2-Butyne : Dimethlyacetylene
Cis and trans configurations of alkenes
-CIS: carbons of parent chain are on the same side of double bond
-TRANS: opposite sides
Cycloalkenes naming
-number double bond in 1 and 2 in the direcrion that gives substituent encountered first the lowest number
-not necessary to indicate location of double bond since matic 1 and 2
-i.e. 4-Ethyl-1-methylcyclohexene
Dienes, Trienes, Polyenes
-Containing more than one double bond: alkadienes, alkatrienes, polyenes
i.e. 2-Methyl-1,3-butadiene
1,3-Cyclopentadiene
-for alkene with “n” C-C double bonds, each can show cis-trans isomerism of:
2^n stereoisomers
Physical properties of alkenes and alkynes
-non polar; London dispersion only
-similar to alkanes having same carbon skeletons
-less dense than water (float)
-soluble to non polar
Terpenes
-compound whose carbon skeleton can be divided into two or more units identical to the five-carbon skeleton of isoprene (2-methyl-1,3-butadiene)
-found in essential oils of plants
Isoprene unit
-carbon 1: head
-carbon 4: tail
-tail of one isoprene unit becomes bonded to the head of another isoprene unit
Characteristic reactions of alkenes
-addition to C-C double bond
-double bond brooen; replace by single bonds form to two new atoms or group of atoms
Addition of hydrogen halides (HYDROHALOGENATION)
-HCl, HBr, HI
Alkene + hydrogen halides ➡️ alkyl halides
Regioselective reaction
-reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions
Vladimir Markovnikov
-Markovnikov’s rule:
In the addition of HX or H2O to an alkene, hydrogen adds to the carbon of the double bond having the GREATER NUMBER OF HYDROGEN
“the rich get richer”
Reaction mechanism
step by step description of how a chemical reaction occurs
Carbocation
-species containing a carbon atom with only three bonds and bearing positive charge
Addition of water: Acid-catalyzed reaction
(HYDRATION)
-catalysis: usually H2SO4
Alkene + H2O ➡️ Alcohol
-follows Markovnikov’s rule
Oxonium ion
-ion in which oxygen is bonded to three other atoms and bears a positive charge
Addition of Bromine and Chlorine
(HALOGENATION)
-addition of halogens to carbon atoms of double bond
Addition of Hydrogen: Reduction
(HYDROGENATION)
Alkenes + H2 ➡️ Alkanes
-in presence of transition metals (Pt, Pd, Ru, Ni)
-aka catalytic reduction/ catalytic hydrogenation
