lecture 6 Flashcards
(31 cards)
how do you calculate the enthalpy change of the reaction?
DH of the bond being broken - DH of the bonds being made
DH is the bond dissociation
what are the two factors that determine whether the enthalpy change is approximately the gibbs free energy change?
- if the entropy change is small
2, if the reaction is in the gas phase
what is an epoxide
an ether in which the oxygen is incorporated into a 3-membered ring
what is peroxy-acid
it is a carboxylic acid with an extra oxygen
what is the reaction between alkenes and peroxy-acids
alkene + peroxy acids –> carboxylic acid
+ epoxide (page 292 pdf)
note the oxygen-oxygen bond in the peroxy acid is weak and can be broken easily
what is the mechanism of the epoxidation of an alkene?
page 293 + my note paper
what is the mechanism of the formation of the intermediate in the addition of bromide to a double bond
page 293 pdf
what are the ways of naming epoxides?
the common name is just adding oxide at the end of the alkenes common name
systemically there are 2 ways
1. the ring is oxirane with the oxygen being in the first position
2. use the alkane name with epoxy as a prefix identifying what carbons are attached to the oxygen
in epoxidation of alkenes what happenes
the double bond geometry is conserved in the epoxide.
what is the hydrogenation of alkenes?
alkene + H2 —> a staturated alkane
palladuim on carbon is used as the catalyst
what happens in the hydrogenation of alkenes
the hydrogens rest on the surface of the catalyst and reacts with the metal atoms, then the alkene approaches the surface of the catalyst which leads to its absorption causing more reactive bonds of both the alkene and the hydrogen then the double bond is replaced by two sigma C-H bonds
note: it is a ‘syn’ mode of addition which where the added hydrogens would end on the same side
what is the relation between the rate of hydrogenation and the stability of the double bond
the slower the rate of hydrogenation and the smaller the heat of hydrogenation is the more stable the double bond.
what increases the stability of a double bond
more substituted alkyl groups on both sides of the C=C double bond
what is the mechanism for the addition of hydrogen halide to isolated dienes and conjugate dienes
lecture 6 slides 11-13
if a conjugate diene is asymmetric then what is the major product
the one with the most stable allylic cation
what is nucleophilic bimolecular substitution Sn2
it is a substitution reaction where the rate is proportional to both the nucleophile and reactant
what are the characteristics of the Sn2 reaction
- the rate is dependent on both the nucleophile and the electrophile
- the stereochemistry of the stereocenter is inverted
- the reaction is slower going from primary halide to tertiary, as well as, less substituted halogenoalkanes to more substituted halogenoalkanes
what is the mechanism that Sn2 reactions follow
it is via a concentrated mechanism (one step) where a transition state is formed.
slide 16.
if an asymmetric center undergoes Sn2 what will happen to the product?
the product would have an inverted configuration meaning that it will go from an S/R to an R/S and it will rotate the plane of plane-polarized light opposite to what the reactant would rotate it
why is the rate of Sn2 reaction decrease as we go from a primary halide to a tertiary halide
since the backside approach of the nucleophile becomes harder and harder due to steric hindrance
what are the factors that affect Sn2 reaction?
- the electrophile (steric hindrance) - less steric hindrance in the organic compound leads to a better electrophile forming when the leaving group leaves
- the leaving group - the better the negative charge is stabilized the better the leaving group, as well as, the weaker the basicity of the group, the better the leaving group.
- the nucleophile - the more negative the group is the better nucleophile it becomes
- the solvent - in protic solvents the larger the size of the anion the more nucleophilic it becomes - in aprotic solvents the smaller the size of the anion the more nucleophilic it becomes
what is protic and aprotic solvents
protic solvent - is a solvent that can easily release a proton and it usually has its acidic hydrogen attached to either oxygen, fluorine, or nitrogen.
aprotic - is a solvent the lacks an acidic proton
why are larger sized anion more nucleophilic in protic solvents
strong solvation leads to nucleophilic shielding but when we have large anions we will have weaker dipole-dipole bonds form between the solvent and the ion, meaning that the larger anion more easily replaces the leaving group while the smaller size anion needs more energy to break from the dipole-dipole bonds between the ion and solvent.
