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CHEM 234 Final Exam with correct answers
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CHEM 234 Final Exam with correct answers
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CHEM 234 Final Exam with correct answers1. organometallic compounds: compounds that contain a metal-carbon bond
2. electronegativity of carbon: 2.5 - not very
3. the greater the difference in electronegativity, the more/less reactive a
compound is: more
4. types of organometallic compounds: 1. strongly polarized carbon-metal bonds
(grignard, Li)
2. less strongly polarized carbon metal bonds (metal is transition metal, ex: Cu)
5. organolithiums: R-Li
- more reactive than grignards
6. grignard reagents: typically an alkyl, vinyl, or aryl-magnesium halide
- usually need to be made in solution from an alkyl halide starting material
- add to carbonyl group (usually aldehyde or ketone)
7. T/F: grignard reagents are very sensitive to water and moisture in air: true!
must flame dry
8. when grignard is in the presence of moisture, it acts as: a base
9. when grignard is in the presence of a carbonyl, it acts as: a nucleophile
10. quenching: stopping reactants from reacting
- aqueous acid + grignard, for example, will stop the grignard because the grignard
will abstract a hydrogen (resulting in Mg + Cl + Br)
11. apparatus of grignard experiment: 1. flame dry
2. semi-dry reaction set up:
- round bottom flask, claisen head, drying tube (with drying agent)
12. claisen head: allows for the addition of reagents in set up while also having a
drying tube in the other portion of the set up. This way, the reaction can be exposed
to air (and remove the moisture) but can also have a seal tight way to add reagents
13. what is the point of using a water bath at the beginning of the grignard
reaction?: grignards are sluggish to start, but run normally after they commence.
14. how to overcome the Mg metal corrosive (oxidized) surface: 1. mechanical
grinding of Mg metal
2. addition of dibromoethane
3. addition of molecular iodine
15. addition of dibromoethane to Mg during grignard synthesis: reacts with the
metal to clean the surface. Ethylene gas forms, so it also allows monitoring of the
reaction
16. addition of molecular iodine to grignard synthesis: chemically cleans the
metal surface
- this was the method used in the experiment
17. adding dry ice to grignard reagent did what: CO2 was the electrophile of the
reaction (carbonyl). As a result, we formed benzoic acid
,18. how did we quench the reaction during grignard synthesis: quenched with
water/HCl
- quenching destroys the grignard because the grignard will react with the dilute acid.
19. liquid-liquid extraction in grignard served to: take the benzoic acid out of the
ether layer by adding NaHCO3 (sodium bicarbonate) --> pH greater than pKa -->
exists in charged form in aqueous layer for extraction
20. at what stage of the isolation process was the bromobenzene separated
from the benzoic acid during grignard synthesis reaction?: the bromobenzene
was separated from the benzoic acid during the liquid-liquid extractions. This is
because bromobenzene exists in the ether layer, and the l-l extraction benzoic acid
into the aqueous layer.
21. at what stage of the isolation process was the magnesium metal separated
from the benzoic acid during grignard?: during the quenching stage. HCl was
added and we waited until all the Mg dissolved (Mg broke apart from the grignard
because the grignard reacted with the dilute acid, leaving a salt of MgBrCl
22. if you have an alcohol with a carbonyl and you attack with a grignard, what
will happen: the grignard acts as a base and deprotonates the alcohol.
23. if you have an alcohol with a carbonyl and you attack with 2 moles of the
grignard, what will happen: the grignard acts as a base first, deprotonating the
alcohol. then, it acts as a nucleophile and attacks the carbonyl.
24. transition metal organometallics: compounds contain bonds between carbon
and the transition metal (d block elements, ex: Fe)
- can get an 18-electron configuration
- achieved through the interaction between the d orbitals of the metal and the hybrid
orbitals of the carbon atom to form bonds with high covalent character because
25. do transition metal organometallics fulfill the octet rule: no; they have
18-electron configuration
- noble gas configuration to the metal and distributes electron density over larger
volume
26. transition metal organometallics high covalent character: achieved through
interaction between d orbitals on the metal and the hybrid orbitals of the carbon atom
- aromatic-type stability
- 6 valence electrons contained in 4s, 4p, and 2 of the 3d orbitals overlap with the
6 filled bonding orbitals of the two cyclopentadienyl rings to form 12 new orbitals, of
which 6 are bonding and filled
- this interaction is only possible in the "sandwich" type interaction where each of
the 10 carbon atoms is equidistant from the iron
, 27. metallocenes: d orbitals of the metal atom interact with the p orbitals of the
aromatic ring
- ex: ferrocene
28. T/F: cyclopentadienyl rings are aromatic: true
29. aromaticity of cyclopentadienyl rings: aromatic
- undergo the reactions typical of other aromatic systems
- major mode of reactivity open to aromatic systems is EAS
30. electrophilic aromatic substitution: electrophile replaces a hydrogen on the
aromatic ring
- substitution because any addition or condensation reactions will cause a loss of
aromaticity!
31. T/F: in electrophilic aromatic substitution, aromaticity is lost: false - it is
maintained, which drives the reaction to completion
32. what type of EAS reaction did we use with the ferrocene reaction: -
friedel-crafts acylation
33. friedel-crafts acylation: the electrophile is generated from an acid chloride
using a lewis acid (such as AlCl3)
- the electrophile formed is the acylium ion and goes on to undergo EAS
34. nucleophile of EAS reaction of acetylferrocene experiment: aromatic ring
35. electrophile of EAS reaction of acetylferrocene experiment: acylium ion
- stabilized by resonance
36. base to help create electrophile of EAS reaction of acetylferrocene exper-
iment: AlCl3
37. acetyl chloride in acetylferrocene experiment: very reactive and strong nu-
cleophile because the C-Cl bond is very polarized
38. why is di-substitution possible in the ferrocene reaction (EAS): because we
have two rings, and only one of them is deactivated after the first substitution
- after the first substitution, the ketone acts as an EWG, which reduces the nucle-
ophilic nature of the aromatic ring and would deactivate it
- Fe electronically isolates the second ring from the ketone, so the second ring is
still able to react
39. T/F: flame drying was necessary for the acetylferrocene experiment: true
- this is because both acetyl chloride and aluminum chloride react with water,
meaning that in order for them to react with each other to generate the electrophile,
it must be dry
40. how did we quench the reactants in the acetylferrocene reaction: used
water to stop the acetyl chloride and aluminum chloride from reacting to generate
the electrophile
- quenching is exothermic!!! this means we need to set up an ice bath
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