Chapter 4 Flashcards
What are alkenes?
hydrocarbons that contain one or more carbon-carbon double bonds?
Size of double bonds?
Double bonds are shorter than single bonds
What are stereoisomers?
Same connectivity but a different arrangement in space?
What is it called if there are two double bonds?
dienes
What is it called if there are three double bonds?
tienes
What are constitutional isomers?
Same molecular formula but different connectivities
Are constitutional isomers also stereoisomers?
Constitutional isomers are not stereoisomers
What is the hybridization of alkene double bond?
Sp2
How much s-character does an sp2 orbital have?
33% = s characterr
67% = p character
Difference between sp3 and sp2 orbitals?
sp2 = electron density is slightly closer to nucleus
- more s-like character = closer to nucleus
What is a pi bond
pi bond = a bond formed by the side overlap of p orbitals
are cis and trans stereoisomers or constitutional isomers?
= stereoisomers because connectivities differ in the spatial arrangement of their atoms
what are cis-trans stereoisomers?
compounds related by an internal rotation of 180 degree about the double bond
alkane vs alkene naming
-ane gets replaced with -ene
How to number chain?
When naming an alkene, we find the longest continuous carbon-carbon chain that includes
the double bond. Then number the chain in the direction that gives the double bond the lowest
number, not the branch position.
What is a Z stereoisomer?
When atoms of higher atomic number are on the same side of the double bond (also cis)
What is an E stereoisomer?
When atoms of higher atomic number are on opposite sides of the double bond (also trans)
- remember E = ears and ears are on opposite sides of your head
How to determine if a stereoisomer is cis or trans?
Determine if the higher priority groups are on the same or opposite sides of the double bond
Rules of determining if a stereoisomer is cis or trans?
1a. assign higher priority to the group containing the atom of higher atomic number
1b. assign higher priority to the group containing the isotope of higher atomic mass
If nothing is determined to move to 2
2. look at the atoms attached and arrange them in descending priority then make pairwise comparisons
If nothing is selected move to 3
3. look at the next set of atoms attached and arrange them in descending priority then make pairwise comparisons
Difference on physical properties of alkanes vs alkenes?
Same physical properties only difference is sp2 vs sp3
What determines the relative stability of alkenes?
more stable alkene = lower heat of combustion
What determines the heat of combustion?
- degree of unsaturation : how many R groups are attached to the alkene
- van der Waals strain : R groups cis to one another are destabilizing
Rank the stability of various R - group substitutes.
Tetra-substitutes > Tri-Substitutes > Di-Substitues > Monosubsitues > no subsitutes
What are exothermic vs endothermic reactions?
Exothermic = heat in a reaction is liberated or released (- change H)
Endothermic = heat in a reaction is absorbed (+ change H)
Which are normally more stable cis or trans stereisomers?
Almost all trans isomers are more stable, due to repulsive forces in cis-isomers
What happens when you can shuffle around bonds?
when you can shuffle around bonds = more stable
What is Ea
Ea = kinetic term
- How fast
- rate = K [reactant 1] [reactant 2]
* the higher Ea, the slower the rxn
-rate-limiting step is the slow step (bigger hill to go up)
What is a catalyst
Catalyst lowers Ea
