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Chapter 13 and 14 | CHEM 342 - Organic Chemistry II, Quizzes of Organic Chemistry

North Dakota State University (NDSU)Organic Chemistry

Class: CHEM 342 - Organic Chemistry II; Subject: Chemistry; University: North Dakota State University-Main Campus; Term: Spring 2011;

Typology: Quizzes

2010/2011

Uploaded on 05/08/2011

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TERM 1
How to determine the structures of molecules
DEFINITION 1
Elemental Analysis atomic compositio n (relative ratios) empirical
formula Mass Spectrometry molecula r formula element
identification (from isotope peaks) Stru ctural features (from
fragments) NMR Spectroscopy Atom Connectivity Functional Group
Identification Stereochemistry Infrare d Spectroscopy Functional
groups UV-VIS Spectroscopy Give infor mation about -conjugated
system
TERM 2
What Kind of States EM Radiations
Correspond to?
DEFINITION 2
electronic- UV-VISvibrational- infrared rotational- microwave
nuclear spin- radofrequency
TERM 3
Mass Spectrometry
DEFINITION 3
Determine the mass of the molecu le Use Nitrogen Rule to identify
whether the peak with the highest m/z ratio is the M+ peak.
Deduce molecular formula using the Rule of 13 MM/13 = n + (r /13)
CnHn+r Fragments - most stable carbo cations (and radicals) are
generated Can give idea about how t he molecule was connected
Can often identify constitutional isom ers Identify isotopes based on
natural abundance ratios
TERM 4
Chemical Shift
()
DEFINITION 4
Chemical shift measures the degree of sh ielding for a nucleus in a
molecule.Protons and 13C in different envi ronments are shielded to
greater or lesser degrees; they have diffe rent chemical shifts. is
measured relative to a standard, trimethy lsilane (TMS), for which the is
set to 0. The value is usually reported in ppm unit to avoid the confusion
caused by using different spectrometer. In a NMR spectrum, the nuclei
that are less shielded will appear at the do wnfield or low field (the left
side) of the spectrum; whereas the peaks for more shielded nuclei appear
at the upfield or high field (right side) of the spectrum.
TERM 5
Effects of Molecular Structure on Chemical
Shifts
DEFINITION 5
Nuclei in different environments experience different degrees of shielding
and have different chemical shifts. Electron egative (EN) substituents
decrease the shielding, leading to large . Th e EN effect is cumulative but
diminishes with distance. Hybridization of ca rbon influence sp2 and sp
hybridized carbons are more deshielded t han sp3 hybridized carbons due
to high EN and the magnetic anisotropy de shielding by the electrons.
However, hydrogens attached to sp hybr idized carbon are more shielded
than those attached to sp2 hybridized carb on due to the magnetic
anisotropy shielding by the triple bond electr ons.
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Download Chapter 13 and 14 | CHEM 342 - Organic Chemistry II and more Quizzes Organic Chemistry in PDF only on Docsity!

How to determine the structures of molecules

Elemental Analysis atomic composition (relative ratios) empirical formula Mass Spectrometry molecular formula element identification (from isotope peaks) Structural features (from fragments) NMR Spectroscopy Atom Connectivity Functional Group Identification Stereochemistry Infrared Spectroscopy Functional groups UV-VIS Spectroscopy Give information about -conjugated system TERM 2

What Kind of States EM Radiations

Correspond to?

DEFINITION 2

electronic- UV-VISvibrational- infrared rotational- microwave

nuclear spin- radofrequency

TERM 3

Mass Spectrometry

DEFINITION 3 Determine the mass of the molecule Use Nitrogen Rule to identify whether the peak with the highest m/z ratio is the M+ peak. Deduce molecular formula using the Rule of 13 MM/13 = n + (r/13) CnHn+r Fragments - most stable carbocations (and radicals) are generated Can give idea about how the molecule was connected Can often identify constitutional isomers Identify isotopes based on natural abundance ratios TERM 4

Chemical Shift

DEFINITION 4 Chemical shift measures the degree of shielding for a nucleus in a molecule.Protons and 13C in different environments are shielded to greater or lesser degrees; they have different chemical shifts. is measured relative to a standard, trimethylsilane (TMS), for which the is set to 0. The value is usually reported in ppm unit to avoid the confusion caused by using different spectrometer. In a NMR spectrum, the nuclei that are less shielded will appear at the downfield or low field (the left side) of the spectrum; whereas the peaks for more shielded nuclei appear at the upfield or high field (right side) of the spectrum. TERM 5

Effects of Molecular Structure on Chemical

Shifts

DEFINITION 5 Nuclei in different environments experience different degrees of shielding and have different chemical shifts. Electronegative (EN) substituents decrease the shielding, leading to large. The EN effect is cumulative but diminishes with distance. Hybridization of carbon influence sp2 and sp hybridized carbons are more deshielded than sp3 hybridized carbons due to high EN and the magnetic anisotropy deshielding by the electrons. However, hydrogens attached to sp hybridized carbon are more shielded than those attached to sp2 hybridized carbon due to the magnetic anisotropy shielding by the triple bond electrons.

1H NMR Spectroscopy

Number of chemically different hydrogens (from the number

of peaks) Relative ratios of protons (from the integration of

peaks) How many neighboring hydrogens (from the splitting

patterns) Functional groups (from the )

TERM 7

Spin-spin splitting in 1H-NMR spectroscopy

DEFINITION 7 Protons on adjacent carbons that are chemically nonequivalent will split each other to n+1 number of peaks (n equals to the number of equivalent Hs on adjacent carbons) Chemically equivalent Hs do not couple with each other. Protons farther away usually do not couple. Two interacting Hs split each other with the same coupling constant (J). Relative ratio of peaks in a multiplet depends on number of spin states of the neighboring hydrogens, which is predicted by the Pascals Triangle. Every splitting can be broken down into a series of doublets. TERM 8

13C NMR Spectroscopy

DEFINITION 8

The number of peaks corresponds to the number of

chemically nonequivalent carbons. The chemical shift values

provide information on functional groups. Symmetry in

molecules can make carbons chemically equivalent.

TERM 9

DEPT 13C NMR Spectroscopy

DEFINITION 9

DEPT-NMR (distortionless enhancement by polarization

transfer) Normal spectrum shows all Cs then: Obtain

spectrum of all Cs except quaternary (broad band decoupled)

Change pulses to obtain separate information for CH2, CH $

CH3 Subtraction reveals each type

TERM 10

UV-Vis spectroscopy

DEFINITION 10

Measures the transitions between electronic energy states.

Provides information on conjugated -electron system. The

HOMO-LUMO energy difference in alkenes decreases (smaller

E) with increased number of alkyl substituents and extended

conjugation, causing max shifts to longer wavelengths.

Carbenes and Their Reaction

Species that contain a divalent carbon (carbon with two bonds and six electrons). Carbenes are very reactive: Normally cannot be isolated and stored; they are intermediates in certain reactions. Carbenes react with alkenes to give cyclopropanes Reaction is stereospecific syn addition CHX3 + KOH --> CX2 +KX + H2O R'- CH=CH-R" + CHX3 --KOH--> Three membered ring TERM 17

Retrosynthetic Analysis for Alcohols

DEFINITION 17 Retrosynthetic analysis is the process by which we plan a synthesis by reasoning backward from the desired product (the "target molecule"). Step 1: Locate the carbon that bears the hydroxyl group. Step 2: Disconnect one of the groups attached to this carbon. What remains are on the carbonyl compound that can be used to prepare the alcohol. The detached part will be on the Grignard reagent.