Lecture 3 Recap Flashcards
(32 cards)
What are the 5 problems with the old structure of benzene (a level just skim over it :) )
- if benzene contains a C=C then it should readily decolorize bromine water in an addition reaction, however it doesn’t and when they don’t react a substitution reaction occurs and this suggest that there is non C=C
- after benzene was discovered, many compounds containing the benzene ring were discovered, along those was dibromobenzene (C2H4Br2), if the kekule structure was correct then there should be 4 isomers of this compound, (page 223) but their are only 3 known to exist with the 2 isomers where the Br is attached to adjacent carbons are identical, but in one isomer the Br are attached with two carbons that have the bond of C–C and the other isomer the Br are attached to the carbons with a C=C bond in-between them, yet there structure is the same this suggest that the bonding between the carbons of the benzene ring are the same. (which goes against the kekule structure)
- data from X-ray diffraction about the lengths of covalent bonds in cyclohexene compared with the data from the covalent bonds of benzene suggest that the carbon-carbon bonds in benzene are all the same and perhaps an intermediate in character between C—C and C=C this goes against the Kekule structure of C-C and C=C being different
- once thermochemical data about enthalpy changes of hydrogenation becomes available another problem occurs. following the data the amount of energy required to add 1 mole of H2 to 1 mol of C=C bonds in cyclohexene(has only one C=C) bond is about 120 Kj mol^-1, and the value for cyclohexa-1,4-diene is double of that since there is two (3ani twice as many) C=C bonds present in Cyclohexa-1,4-dienen. now benzene following the kekule structure can be named as Cyclohexa-1,3,5-triene so following that trend it should have an enthalpy change of hydrogenation triple that of cyclohexene since it has triple the amount of C=C bonds but in reality Benzene has a much lower enthalpy change going against the kekule structure
- the infrared structure of cyclohexene and benzene show that cyclohexene has a absorption around 1650 which is typical to that off a alkene C=C stretch but Benzene has a stretch around 1500, 1580, 1450 which is typical of a aromatic showing that the carbon bonds of Benzene don’t fit the trend
Explain the bonding in Benzene
the carbons in benzene all are sp^2 hybridized, giving each bond an angle of 120 which forms a hexane planer, due to this structure of the planer the unused P orbitals of all the carbons are parallel to each other causing p orbital overlap between all of them meaning that the six pi electrons (one in each p) are shared by all of the carbons in the structure making them have a delocalized form with the electrons roaming freely in the compound
What is a resonance contributor? And a resonance hybrid
Resonance contributor -One set of Lewis structures that differs only in the distribution of electrons. (Resonance contributors are imaginary)
Resonance hybrid - a compound that experiences resonance(delocalization) where it is represented in a written form as an average of 2 or more resonance contributors separated from each other with a double-headed arrow ( NOT THE EQ ARROW)
y3ani benzene is an example of resonance hybrid if still confused (8.3)
Electron delocalization is most effective when
the atoms sharing the delocalized electrons lie in the same plane so their p-orbitals can maximally overlap. for example, a structure like cyclooctatetraene has it’s atoms with a used p orbital in different planes so it is only able to form localized pi bonds
what is the structure of NOO or COO etc
so in such structure the bond length of the oxygens respectively with N/C is the same length even thou we would usually draw it has N=O and N-O but that is not accurate so we can represent the proper structure of such compounds by drawing the two resonance contributors between them a double-headed arrow and the resonance structure which same bonds length for both N/C - O bonds and the same shared negative charge on the Oxygens
Usually, delocalized electrons result from
the overlapping of a P orbital with 2 adjacent P orbitals (y3ani how in benzene the p orbital on one carbon overlaps the p orbital of two adjacent carbon atoms)
What are the rules for drawing resonance contributors?
first, draw one Lewis structure of a resonance contributor, then to draw the rest follow these rules:
■ Only electrons move. Atoms never move.
■ Only p electrons (electrons in p bonds) and lone-pair electrons can move. (Never moves electrons.)
■ The total number of electrons in the molecule does not change. Therefore, each of the resonance contributors for a particular compound must have the same net charge.
Important to note that electrons cannot move to an sp^3 hybridized carbon as it already has a full octet. Electrons usually move to sp^2 (which is either carbonation or C=C) or sp which is just a carbon triple bond
When drawing the resonance hybrid always remember
to make the pi bonds that move around dashed lines on top of the already present sigma bond and the negative for positive charged should/might be shared
the greater the predicted stability of a resonance contributor
the more it contributes to the structure of the resonance hybrid, which means the more similar the contributor is to the real hybrid
The relative stability of a resonance contributor decreases with?
- When there is an atom with an incomplete octet
- When a negative charge is present at not the most
electronegative atom - When a positive charge is present on an electronegative atom
- When there is charge separation (because energy is required to keep the charges separate)
(note that an incomplete octet makes the structure more unstable than the other 3 features)
why do we sometimes move electrons away from the electronegative atom when drawing a resonance contributor
movement of electrons even if away from the electron negative atom is better than no moment at all as electron delocalization makes the compound more stable.
the stability of positive charge and carbons
the positive charge on a tertiary carbon would make the resonance contributor more stable than the positive charge of a secondary carbon. Following (Teritatry > Secondary > Primary) as tertiary is more stable than secondary which secondary is more stable than primary
what is delocalization energy
the extra stability a compound gains from having delocalized electrons ( this is also called resonance energy)
The delocalization energy of a compound that has delocalized electrons depends on?
the number and predicted stability of the resonance contributors where the greater the number of relatively stable resonance contributors the greater the delocalization energy.
Also, it depends on the equivalency of the structure of the resonance contributors where the more nearly equivalent structure of the resonance contributors is the greater the delocalization energy
what are isolated dienes and conjugated dienes
isolated dienes - have an isolated double bond where they are separated by more than one single(sigma) bond
conjugated dienes - have a conjugated double bond where they are separated by 1 single(sigma) bond
the most stable alkene has the smallest
heat of hydrogenation
which is more stable isolated dienes or conjugate dienes according to the heat of hydrogenation
conjugate dienes as they have smaller heat of hydrogenation
what are the two factors that make conjugate dienes more stable than isolated dienes
- the first factor is delocalized electrons. The pi electrons in each of the double bonds in isolated dienes are localized between the two carbons ( since attached to a sp^3 carbon) while the pi electrons in conjugated dienes are delocalized between the carbons which increases the compound’s stability.
- the second factor is the hybridization of the orbitals that form the carbon-carbon bond as the 2s orbital, in general, is closer to the nucleus than the 2p orbital, so a bond formed between a sp^2-sp^2 is stronger and shorter than a bond between a sp^3-sp^3 that is also due to sp^2 have a more s character thus one single bond in the conjugate dienes is stronger than all the other single bonds in isolated dienes so the more stable the conjugated compound is
what are allenes
compounds that have cumulated double bonds. Its central carbon is sp hybridized forming two pi bonds by the overlapping of its two p orbitals with the p orbitals of the neighboring sp^2 carbons
in order for an organic compound to conduct electricity it must
have delocalized electrons within its structure (y3ani throughout all of its structure not just in the middle)
What is allylic/Benzylic cation
allylic cation: it is a carbocation with a positive charge on an allylic carbon( which is a carbon adjacent to a sp^2 hybridized carbon in an alkene)
Benzylic cation: it is a carbonation with a positive charge on the benzylic carbon (which is a carbon adjacent to a sp^2 hybridized carbon in a benzene ring)
what is an allyl cation/benzyl cation
allyl - an unsubstituted allylic cation
benzyl - an unsubstituted benzylic cation
why are allylic and benzylic cations more stable than the rest of the cations
because the benzylic cation forms a 5 resonance structure and the allylic cations form 2 so due to their delocalizations they are more stable (relatively equal to a tertiary carbonation)
Note: similar to normal carbocations the tertiary of benzylic/allylic carbonation is more stable than the secondary which us more stable than the primary.
Why is benzene so stable
because it is an aromatic compound with delocalized electrons that forms 2 identical resonance contributors
