CHEM 130 Lecture Notes - Lecture 7: Proton Nuclear Magnetic Resonance, Ring Current, Local Field

23 views1 pages

oliverabbit203

27 Aug 2016

School

Department

Course

Professor

For unlimited access to Class Notes, a Class+ subscription is required.

Document Summary

In 13c nmr, one region was enough for both of these, but see how different things are with proton nmr. The two carbon signals are almost the same (1. 3 p. p. m. difference < 1% of the total. 200 p. p. m. scale) but the proton signals are very different (1. 6 p. p. m. difference = 16% of the 10 p. p. m. scale). There must be a fundamental reason for this. The benzene ring current causes large shifts for aromatic protons. The benzene ring looks similar at first sight, and the plane of the molecule is indeed a node for all the p orbitals. However, benzene is aromatic" it has extra stability because the six p electrons fit into three very stable orbitals and are delocalized round the whole ring. The applied field sets up a ring current in these delocalized electrons that produces a local field rather like the field produced by the electrons around a nucleus.

Related Questions

Shielding is due to the electrons that surround atomic nuclei in molecules. As electrons spin, they oppose (shield nuclei) the external magnetic fields in directions perpendicular to the spin but augment (increase, deshield) the external magnetic fields in the plane of the spin. This difference in magnetic effect of electron spin due to orientation is called anisotropy (not equal regions).

Electronegativity indirectly affects anisotropy because groups with less electrons around them are more deshielded. But anisotropy is greatly observed in systems like benzene where being in the plane of the ring is deshielding but being on top of the plane of the ring is highly shielding. Because of anisotropy, tables of where NMR signals are located may be way off.

In which direction does shielding increase in a NMR spectrum?

Answer choices:
A) left
B) up
C) down
D) right

1. Of Huckel's requirements for aromaticity, only this one is NOT required in the case of some compounds to be considered aromatic? 8. Which sequence of reagents would produce b) The system must be monocyclic. c) There must be (4n + 2) Ï electrons. d) The Huckel number of electrons must be completely 2. Zn(Hg), conc. HC delocalized. 2. 4-amino-2-chlorobenzaldehyde is NOz H2N NO2 3. Which of the following compounds are aromatic? Hint: 9. Refer to the following list of possible answers: So ) halogen h) RC-O D diazonium ion a. What group weakly activates benzene to b. What group slightly deactivates benzene to SeAr because a) I and II only c)I, II and â ¢ b) II and III only d) I and IlI only SAr by a +I e-donor inductive effect? of its-I withdrawal inductive effect, but is an ortho-/para- director because of its +R resonance e-donor 4. Which one of the following compounds is anti-aromatie? c. What group inhibits Friedel-Crafts alkylation and acylation d. What species is the electrophile e. What species is formed when but strongly activates a halo benzene to S&Ar because of its strong -R e-withdrawal resonance effect? In which case is the indicated unshared pair of electrons Nora contributor to the Ï aromatic system? 5. in Friedel-Crafts acylation? aniline reacts with NaNO HCVS? 10. Arrange these benzene compounds in order of decreasing (fastest to slowest) SeAr reactivity: . Cyclopentadiene is unusually acidic for a hydrocarbon a) The carbon atoms are all sp hybridized. b) Cyclopentadiene is aromatic. c) Removal of a proton from cyclopentadiene yields an a) III > IV > 11 > 1 c) IV I> III d) IV > â ¡ > I> aromatic anion. d) Removal of a hydride ion from cyclopentadiene What might be predicted to happen when the following substance undergoes Friedel-Crafts acylation? produces an aromatic cation. 11. Recalling that benzene has a resonance energy of 152 k mol and naphthalene has a resonance energy of 255 k mol', predict the two positions which would be occupied by bromine when phenanthrene (below) undergoes addition of Brã (circle two numbers). a) Substitution occurs in ring A, o- to the methylene gro b) Substitution occurs in ring A, o-to the nitro group. c) Substitution occurs in ring A, m- to the methylene g d) Substitution occurs in ring B, o-/p- to the methylene e) Substitution occurs in ring B, m. to the methylene 10 9

For each of the following, fill in the blank with the correct response. All of these fill-in-the-blank problems pertain to material covered in the sections on alkanes, alkenes and alkynes, aromatic hydrocarbons, and hydrocarbon derivatives.

a. The first "organic" compound to be synthesized in the laboratory, rather than being isolated from nature, was _ _ _ _ _, which was prepared from _ _ _ _ _.

b. An organic compound whose carbon-carbon bonds are all single bonds is said to be _ _ _ _ _.

c. The general orientation of the four pairs of electrons around the carbon atoms in alkanes is _ _ _ _ _.

d. Alkanes in which the carbon atoms form a single unbranched chain are said to be _ _ _ _ _ alkanes.

e. Structural isomerism occurs when two molecules have the same number of each type of atom but exhibit a different arrangement of the _ _ _ _ _ between those atoms.

f. The systematic names of all saturated hydrocarbons have the ending _ _ _ _ _ added to a root name that indicates the number of carbon atoms in the molecule.

g. For branched hydrocarbon, the root name for the hydrocarbon comes from the number of carbon atoms in the _ _ _ _ _ continuous chain in the molecule.

h. The positions of substituents along the hydrocarbon framework of a molecule are indicated by the _ _ _ _ _ of the carbon atom to which the substituents are attached.

i. The major use of alkanes has been in _ _ _ _ _ reactions, as a source of heat and light.

j. With very reactive agents, such as the halogen elements, alkanes undergo _ _ _ _ _ reactions, whereby a new atom replaces one or more hydrogen atoms of the alkane.

k. Alkenes and alkynes are characterized by their ability to undergo rapid, complete _ _ _ _ _ reactions, by which other atoms attach themselves to the carbon atoms of the double or triple bond.

l. Unsaturated fats may be converted to saturated fats by the process of _ _ _ _ _.

m. Benzene is the parent member of the group of hydrocarbons called _ _ _ _ _ hydrocarbons.

n. An atom or group of atoms that imparts new and characteristic properties to an organic molecule is called a _ _ _ _ _ group.

o. A _ _ _ _ _ alcohol is one in which there is only one hydrocarbon group attached to the carbon atom holding the hydroxyl group.

p. The simplest alcohol, methanol, prepared industrially by the hydrogenation of _ _ _ _ _.

q. Ethanol is commonly prepared by the _ _ _ _ _ of certain sugars by yeast.

r. Both aldehydes and ketones contain the _ _ _ _ _ group, but they differ in where this group occurs along the hydrocarbon chain.

s. Aldehydes and ketones can be prepared by _ _ _ _ _ of the corresponding alcohol.

t. Organic acids, which contain the _ _ _ _ _ group, are typically weak acids.

u. The typical sweet-smelling compounds called _ _ _ _ _ result from the condensation reaction of organic acid with an _ _ _ _ _.

rubyzebra85

CHEM 130 Lecture Notes - Lecture 7: Proton Nuclear Magnetic Resonance, Ring Current, Local Field

Document Summary

Get access

Related textbook solutions

Chemistry: Structure and Properties

Basic Chemistry

Principles of Chemistry Molecular Approach

Chemistry: Structure and Properties

Principles of Chemistry Molecular Approach

Chemistry: A Molecular Approach

Chemistry: A Molecular Approach

Principles of Chemistry: A Molecular Approach

Chemistry: The Central Science

Related Documents

CHEM 130 Lecture Notes - Lecture 6: Aldehyde, Aliphatic Compound, Ring Current

CHEM 130 Lecture Notes - Lecture 10: Carbon-13 Nuclear Magnetic Resonance, Heterocyclic Compound, Cytosine

CHEM 130 Lecture Notes - Lecture 8: Alkoxy Group, Sulfone, Nucleophilic Addition

Related Questions