Exam 2 Flashcards by Haley Thompson

Electromagnetic Radiation

light has both wave like and particle like properties

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What is wavelength?

distance from peak to peak
- smaller wavelength = higher energy

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What is frequency?

time to get from one peak to the next, higher frequency = higher energy

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Which frequency of light has the highest energy?

gamma rays

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Which frequency of light has the lowest energy?

microwave radio

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What information about a molecule can be determined by nuclear magnetic resonance (NMR)?

the specific arrangement of all carbon and hydrogen atoms in the compound
- radio waves

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What information about a molecule can be determined by IR spectroscopy?

The functional groups present in the compound
- infrared

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What information about a molecule can be determined by UV-VIS spectroscopy?

any conjugated pi system in the compound
- helps us measure concentration
- visible and ultraviolet

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What light measures excitation of electrons?

UV/Vis

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What light measures excitation of bond? (vibration excitation)

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What light measures rotation of molecules?

IR/NMR

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What is the result of covalent bonds absorbing energy in the IR region?

molecular vibration (bending and stretching of the bond)

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What determines the quantized energy of a bond?

bond type (single, double, or triple)
atoms attached

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What does a higher wavenumber indicate?

higher energy

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What is the wavenumber for a C-H bond?

~3000 cm−1

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What is the wavenumber for a C-D bond?

~2200 cm-1

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What is the wavenumber for a C-O bond?

~ 1100 cm-1

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What is the wavenumber for a C-Cl bond?

~700 cm-1

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What is the wavenumber for a C///N bond?

~2200 cm-1

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What is the wavenumber for a C//N bond?

~1600 cm-1

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What is the wavenumber for a C-N bond?

~1100 cm-1

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What is the relationship between atom size, energy and wavenumber?

smaller atoms absorb higher energy/ higher wavenumber

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What is the relationship between bond strength, energy, and wavenumber?

stronger bonds absorb higher energy/ higher wavenumber

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Wavenumber of sp3 (alkane) hybridized carbon?

~2900 cm-1

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Wavenumber of sp2 (alkene) hybridized carbon?

~3100 cm-1

Wavenumber of sp (alkyne) hybridized carbon?

~3300 cm-1

What is the effect of resonance on wavenumber?

resonance usually decreases the energy level of a bond by delocalizing electrons

What is the wave number of a ketone?

~1720 cm-1

What is the wavenumber of a conjugated ketone?

~1680 cm-1

Size of C-H peaks?

typically large

Efficiency in absorbing the IR radiation (dipole moment)

strong dipole = strong peak

Result of bond with stronger dipole moment?

stronger peaks in the IR spectrum - symmetrical molecule has no stretching peak

What can lead to the broadening of a peak in an IR spectrum?

hydrogen bonding

Size of peak for an alcohol diluted in a solvent that does not hydrogen bond?

peak is narrow

Size of peak for an alcohol diluted in a solvent that does hydrogen bonding?

peak is broad

How many peaks do primary amines have?

2 peaks

How many peaks do secondary amines have?

1 peak

What is mass spectroscopy?

- doesn't use electromagnetic radiation - bombards sample with electrons

What is the primary use of mass spectroscopy?

to determine molecular weight and molecular formula

How does a mass spectrometer work?

- a compound is vaporized, then ionized, and undergoes fragmentation - the masses of the ions are detected and graphed

Molecular Ion (M)*` - The radical cation

- +1 charge on the radical cation - no change in mass - splits into an uncharged radical and carbocation (fragmentation) - sorted by mass to charge (m/z) ratio

What is the base peak?

tallest peak

(M+1)*` Peak

- Due to heavier isotopes of common atoms like carbon

(M+2)*` Peak

- most often due to the presence of Cl or Br - second most abundant forms of Cl and Br have +2 mass

What is NMR spectroscopy

Nuclear magnetic resonance spectroscopy - most powerful technique for investigating organic compounds - Interaction between electromagnetic radiation and atomic nuclei

Which nuclei have a net nuclear spin?

- nuclei with an odd number of protons and or an odd number of neutrons

What is the result of nuclear spin?

creates a tiny magnetic field = magnetic moment

When a strong external magnetic field is applied, the nuclei will align with or against the external field. Aligning with = Aligning against=

Aligning with = alpha spin Aligning against= beta spin

What does the energy difference between the alpha and beta states depends on:

- strength of external magnetic fields - The electronic environment of the nuclei

Is the alpha or beta spin state higher energy?

beta state

What are the 4 key characteristics of the 1H NMR Spectrum

1. # of signals 2. chemical shifts 3. integration 4. shape/ multiplicity

Number of signals =

number of chemically distinct protons

Enantiopic Protons

Existence of a plane of symmetry means the H are equivalent

Diastereotopic Protons

Not equivalent - no plane of symmetry - can't rotate *only when chiral center

What is the equation for chemical shift?

observed shift from TMS in hertz / operating frequency of the instrument in hertz

What happens to the chemical shift value when electron withdrawing groups are added to the atom?

It gets bigger

How much does the chemical shift value increase with the oxygen of an alcohol or an ether?

+2.5

How much does the chemical shift increase for oxygen of an ester

How much does chemical shift increase for a carbonyl group (C=O)?

Multiplicity/ splitting patern?

the number of peaks in a given signal

Coupling constant/ J value?

the degree to which a neighboring proton will couple to its neighbor

What is a diene?

compound with two double bonds

Conjugated pi system

- double bonds separated by one sigma bond

Is a trans or a cis diene more stable

HOMO

highest occupied molecular orbital

LUMO

lowest occupied molecular orbital

1,2-Adduct

kinetic product

1,4-Adduct

thermodynamic product

What are the types of pericyclic reactions?

- electrocyclic - cycloaddition - sigma tropic rearrangement

Cycloaddition

2 reactants --> 1 ring structure

Electrocyclic

open chain ---> cyclic structure

sigmatropci rearrangement

cyclic rearrange but no ring

Diels-Alder reaction

- a [4+2] cycloaddition reaction - mechanism is cyclic and concerted

Thermodynamics of Diels-Alder

- forward reaction favored between 0-200C - Temperature >200 favors the reverse reaction (retro Diels-Alder)

Diels- Alder Reactants

Diene + Dienophile - Reaction is slow unless dienophile has an electron withdrawing group

Stereospecificity of Diels- Alder

- Diene must be in cis conformation

Is the exo or endo position preferred?

endo (pointed down)

Exam 2 Flashcards

(77 cards)