Organic Chemistry 2 - Exam 1 (Ch. 10-11)

Hydrocarbon functional groups

• alkane, alkene, alkyne, benzene ring

• non-polar

• extremely weak acids

• low boiling points

• only intermolecular interactions are London dispersion forces

• Carbon electronegativity: 2.5; Hydrogen electronegativity: 2.2

Alkane

• hydrocarbons containing no multiple bonds

• substituents are called alkyl groups

• common examples: methane, ethane, propane, butane, and octane

• very non-polar; C-H bond is highly covalent

• sp3 hybridized and have tetrahedral geometry about the carbon

Alkene

• hydrocarbons with one or more carbon-carbon double bonds

• examples: ethene, propene, and butene

• substituents are called alkenyl groups (-vinyl is often used to refer to -CH=CH2)

• carbons are sp2 hybridized, with a trigonal planar geometry

Alkyne

• contain a carbon-carbon triple bond

• sometimes called acetylenes

• substituents are called alkynyl groups

• carbons are sp hybridized with a linear geometry

Benzene ring

• six-membered ring containing 3 double bonds

• aromaticity property —> makes them unusually stable

• substituents are called phenol groups

• carbons are sp2 hybridized with trigonal planar geometry

Amine

• contains the functional group -NH₂, -NHR, or NR₂ (R is a hydrocarbon)

• substituents are known as amino groups

• examples: morphine, codeine, cocaine

• amines with N-H bonds are capable of hydrogen bonding —> leads to higher boiling points and water solubility

• lone pair on the nitrogen can act as a base

Alcohol

• R-OH —> contains carbon bonded to the hydroxyl group -OH

• examples: methanol, propanol

• O-H bond is highly polarized and participates in hydrogen bonding

• hydroxyl groups also increase water solubility

• weak acids; can also act as Lewis bases

Ether

• R-O-R —> oxygen atom flanked by two bonds to carbon

• commonly used as lab solvents

• examples: diethyl ether, tetrahydrofuran, dioxane

• cannot serve as hydrogen-bond donor

Alkyl halides

• R-X —> alkyl group bonded to a halide (F, Cl, Br, I)

• examples: bromobutane, methyl bromide, chloroform

• dipole-dipole interactions lead to higher boiling points than those found in alkanes

• if R is alkene, they are alkenyl halides

• very important functional groups for substitution and elimination reactions

Thiol (mercaptan)

• R-SH

• sulfur atom is not nearly as electronegative as oxygen, so the S-H bond is considerably less polarized

• can act as weak acids, but stronger than alcohols

• most notorious for their strong odor

Carbonyl functional groups

• contain C=O bond

• found in aldehydes, ketones, esters, and carboxylic acids

• C=O bond is strongly polarized towards oxygen and the carbon bears a partial positive charge

Aldehydes

• RCHO —> have C=O bonded to a carbon and to C-H

• examples: formaldehyde, acetaldehyde, benzaldehyde

• have polar covalent bonding but are not hydrogen bond donors

Ketones

• RC(O)R —> have C=O bonded to two carbons

• example: acetone

Esters

• RCOOR —> similar to carboxylic acids, except the O-H bond is replaced with an O-C bond

• contain polar bonds, but do not participate in hydrogen bonding

• notable for their sweet smells

Carboxylic acids

• RCOOH —> have a carbonyl bonded to -OH, but are distinct functional groups from alcohols

• examples: acetic acid (vinegar), formic acid, butanoic acid

• hydroxyl group participates in hydrogen bonding—> higher boiling points

• tend to be relatively weak acids, not undergoing full dissociation in water

Amides

• contain a carbonyl carbon attached to an amino group

• amino acids linked together through formation of an amide are known as peptides

• amides containing N-H bonds can participate in hydrogen bonding

Nitriles

• carboxylic acid derivatives —> can be formed by dehydration of amides

• common solvent is acetonitrile

• -CN substituent is sometimes referred to as a cyanide; undergoes reactions with alkyl halides

Acid halides

have -OH replaced with F, Cl, Br, or I

Anhydrides

• contain an oxygen flanked by two carbonyls

• can be formed from two equivalents of a carboxylic acid with accompanying loss of H2O

Epoxides

• technically a type of ether

• however, they participate in a number of reactions that ethers don’t

Spectroscopy

• spectroscopy is a technique for analyzing the structure of molecules, usually based on differences in how they absorb electromagnetic radiation

• four main types: nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, ultraviolet (UV) spectroscopy, and mass spectrometry (MS).

NMR spectroscopy

• two types to know, ¹H NMR and ¹³C NMR

• probes the vicinity of individual nuclei, particularly hydrogens and carbons, and provides the most detailed information regarding the atomic connectivity of a molecule

Chemical shift

• the difference between the resonant frequency of an isotope’s spinning protons and the signal of the reference model

• in NMR spectroscopy, it is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field

• the position and number of chemical shifts are often diagnostic of the structure of a molecule

Shielding

occurs when there are more electrons around the nucleus of an atom, which creates a larger opposing magnetic field

¹H NMR

Gives information about:

• chemical shift

• integration

• spin-spin splitting

¹³C NMR

Gives information about:

• chemical shift

• DEPT

Constitutional (structural) isomers

compounds that have the same molecular formula but different bonding patterns

Diastereomers

• two molecules which are stereoisomers —> same molecular formula, same connectivity, but different arrangement of atoms in space

• more than one chiral center —> one in same configuration, one in opposite configuration

Enantiomers

a pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed one upon the other

Integration

• in NMR, it is a measure of the area of the peaks in the spectra

• the area of the peak is proportional to the number of atoms that it represents

Coupling constant, J

• a measure of the spin-spin coupling effect between two protons in a molecule

• expressed in hertz (Hz) and is the difference between two adjacent sub-peaks in a split signal

• to calculate, convert the peaks from ppm to hertz and find the difference

geminal coupling

the coupling of two hydrogen atoms on the same carbon atom

vicinal coupling

• a type of coupling that occurs between hydrogen atoms on adjacent carbon atoms

• represented by ³J, since they couple through three bonds

DEPT

• Distortionless Enhancement by Polarization Transfer

• used in ¹³C NMR spectroscopy to distinguish between a CH3 group, a CH2 group, and a CH group

Infrared (IR) Spectroscopy

Mass spectrometry (MS)

• the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection

• used to study and identify chemical substances or functional groups

vibrational excitation

a mechanical mechanism that causes the vibration of particles, such as HCl, during collisions with a surface

M+ molecular ion

• the radical ion produced when the molecule is ionized by loss of an electron from the molecule

• the m/z (mass-to-charge ratio) of this ion corresponds to the molecular weight (MW) of the sample

• # of atoms x molecular weight = m/z

base peak

• the tallest peak in a mass spectrum

• represents the most common and most abundant ion —> most likely ion to form

• assigned 100% intensity

degree of unsaturation

• a calculation that determines the number of rings and multiple bonds in an organic compound

• also known as the index of hydrogen deficiency (IHD)

• DoU = 0 —> only single bonds, no rings

• each ring or double bond counts as one DoU

• triple bonds count as two degrees of unsaturation

What information is primarily obtained from nuclear magnetic resonance spectroscopy?

arrangement of carbon and hydrogen atoms in a compound

What information does the intensity (integration) of a signal in the ¹H NMR spectrum give you?

A ratio for the number of hydrogens that give rise to the signal

The separation of ions in the mass spectrometer is done by their___?

Mass to charge ratio

In an IR spectrum, what does a strong absorbance peak at 1720 cm^-1 indicate?

a carbonyl group, C=O

Which information is primarily obtained from infrared spectroscopy?

the functional groups present in a compound

What is the “nitrogen rule” in mass spectrometry?

• organic molecules composed of C, H, O, and halogens have even molecular weights

• since N forms 3 bonds, compounds with an odd number of nitrogen atoms have odd molecular weights

Resonance energy

• resonance structures refers to the various arrangements of electrons among the atoms in a molecule

• stretching frequency rises as bond strength and length decrease

• a molecule is more stable the more resonance structures it has

Why are ¹³C spectra decoupled?

• splitting patterns would be too complex without proton decoupling

  • broadband decoupling is used to suppress proton coupling and simplify the spectrum

Why is splitting not observed between adjacent carbon atoms in ¹³C spectra?

only 1% of carbon exists as the NMR active nuclei ¹³C

What is a wavenumber?

the inverse of a wavelength

What region is associated with O-H bond stretches?

3200—3600 cm^-1

Geminal coupling constant

• geminal hydrogens (attached to the same carbon)

• 0 to 2 Hz

Vicinal coupling constant

• proton sets on nearby sp3 hybridized carbons

  • 6 to 8 Hz

Cis coupling constant

• hydrogen atoms in cis configuration

• 5 to 12 Hz

Trans coupling constant

• hydrogen atoms in trans formation

• 11 to 18 Hz

What region corresponds to the stretching vibration of the C=C bond?

1600—1800 cm^-1

Mass Spectrometry: Chlorine

• isotopes: ^{35}Cl and ^{37}Cl

• M+ peak will have a corresponding M+2 peak approximately 1/3 the intensity of the M+ peak

Mass Spectrometry: Sulfur

• isotopes: ^{32}S, ^{33}S, and ^{34}S

• M+ peak will have a corresponding M+2 peak that is about 4% of the intensity of the M+ peak

Mass Spectrometry: Bromine

• isotopes: ^{79}Br and ^{81}Br

• the M+ peak will have a corresponding M+2 peak that is approximately equal in intensity (100%) to the M+

Mass Spectrometry: Alcohol (O-H group)

• typically show a loss of water (18 Da) from molecular ion

• M+ peak —> can often be very small

• prominent M-18 peak

• cleavage of C-C bond next to the oxygen usually occurs

Mass Spectrometry: Aldehyde

cleavage of bonds next to the carboxyl group results in the loss of hydrogen (-1) or the loss of CHO (-29)

Mass Spectrometry: Alkane

• molecular ion peaks are present, possibly with low intensity

• fragmentation pattern contains clusters of peaks 14 mass units apart

Mass Spectrometry: Amides/Amines

• primary amides show a base peak

• for amines, molecular ion peak is an odd number

Mass Spectrometry: Aromatic compounds

molecular ion peaks are strong due to the stable ring structure

Mass Spectrometry: Esters

• fragments appear due to bond cleavage next to C=O (alkoxy group loss, -OR)

• also appear due to hydrogen rearrangements

Mass Spectrometry: Ketones

major fragmentation peaks result from cleavage of the C-C bonds adjacent to the carbonyl