Survey of the Natural Sciences Study Guide for the DAT

Organic Chemistry

The organic chemistry portion of the Survey of the Natural Sciences comprises 30 questions. This section will cover everything from the mechanisms and basics of organic chemistry reactions to chemical separations and laboratory procedures.

Mechanisms

Mechanisms detail the steps of a chemical reaction, thereby explaining how reactants become products. Through mechanisms, we can see how electrons move and how bonds are broken and formed. Having a thorough understanding of the mechanisms of a chemical reaction allows us to predict and design reactions. This section will briefly detail some reaction types and the steps they follow.

Elimination

Elimination reactions are when a single molecule is broken down into two components.

Addition

Addition reactions are when two molecules are added together to make a new molecule with no atoms left over.

Images retrieved from: https://openstax.org/books/organic-chemistry/pages/6-1-kinds-of-organic-reactions

Free Radical

A free radical (or just radical) molecule is highly reactive because it lacks a complete valence shell of electrons. A free radical may take an electron from another reactant, resulting in a new radical.

Retrieved from: https://openstax.org/books/organic-chemistry/pages/6-6-radical-reactions

Substitution Mechanisms

Substitution reactions are when two reactants interact to form two new products via atom rearrangement.

Retrieved from: https://openstax.org/books/organic-chemistry/pages/6-1-kinds-of-organic-reactions

Other Mechanisms and Reactions

Aromatic substitution reactions are when a hydrogen on an aromatic ring (a six-membered carbon ring with alternating single and double bonds) is replaced by an electrophile via nitration, sulfonation, halogenation, or acylation. Oxidation reactions increase the number of bonds to electronegative atoms like oxygen. Reduction reactions increase the number of bonds to hydrogen. Carbocation rearrangements occur when a hydride or alkyl group shifts to form a more stable carbocation.

Important Note about Mechanisms

On the DAT, organic chemistry mechanisms are represented by curved arrows.

The movement of electron pairs is shown with a double-headed arrow. The movement of a single electron is shown with a single-headed arrow.

Retrieved from: https://openstax.org/books/organic-chemistry/pages/6-2-how-organic-reactions-occur-mechanisms

With these types of images as a reference, you will need to be able to:

• predict products based on starting materials and curved arrows
• predict curved arrows based on starting materials and products

The Chemical and Physical Properties of Molecules

Molecules and the chemical groups present in them yield their unique qualities and reactivities. By understanding these qualities, methods were developed for identifying functional groups in a given solution and separating a solution into individual components.

Spectroscopy

In spectroscopy, a given molecule will absorb or emit light. This type of measurement is unique to a given molecule based on its structure and can thus be used to identify an unknown substance.

1H NMR

This type of spectroscopy involves using magnetics to align the magnetic moments of nuclei and exposing them to electromagnetic radiation to excite all nuclei to the β-state (higher energy). The resulting peak locations of the resonance absorption pattern indicates which substance is being tested.

13C NMR

This type of spectroscopy is similar to 1H NMR, but a larger sample is needed. Additionally, carbons do not tend to couple. 13C NMR can be recorded without coupling via spin decoupling.

Infrared

Infrared (IR) spectroscopy is a measurement of the vibrations of molecules. Functional groups present in a molecule are revealed in a fingerprint region.

Multi-Spectra

Multi-spectra spectroscopy uses multiple spectral ranges to analyze the composition of samples. Information from different parts of the electromagnetic spectrum is combined to better identify characteristics of the sample being studied.

Structure

Each functional group has distinct characteristics that guide a molecule’s interaction with its environment.

Polarity

Polarity describes how the electrons are shared in a molecule and thus how the molecule’s electrical charge is distributed. When electrons are shared unequally, a dipole moment is created, which makes a polar molecule. Polar molecules interact better with other polar molecules, and nonpolar molecules interact better with other nonpolar molecules. The degree of polarity is important in understanding solubilities, boiling points, melting points, and chemical reactivities.

Intermolecular Forces

The intermolecular forces between molecules determine their physical properties. In decreasing order of strength, hydrogen bonding, dipole-dipole interactions, and London dispersion forces are all types of intermolecular forces.

solubility—At a given temperature and pressure, solubility describes the ability of a solute to dissolve in a solvent.

melting and boiling points—The melting point is the temperature at which a solid becomes a liquid, and the boiling point is the temperature at which a liquid becomes a gas. The stronger the intermolecular forces, the greater the temperatures required to change the phase of matter.

Laboratory Theory and Techniques

Many laboratory techniques can be employed to identify individual components of a solution. The proper technique must be selected based on the qualities of the solution elements, including their charge, polarity, and mass.

Thin Layer Chromatography (TLC)

TLC uses a mobile phase and a stationary phase to separate individual compounds present in a solution. The distance traveled is known as the Rf value and depends on the polarity, affinity for the TLC plate, and affinity for the solvent.

Separations

To separate a given solute from a solution, many techniques can be attempted. Extraction is a method in which solutes are separated from one another based on aqueous or organic qualities. Filtration is a method used to separate a solid from a liquid. Recrystallization dissolves an impure crystal in hot solvent then cools it to separate the pure crystal from the impurities. Sublimation transfers a solid directly to a gas without it having to pass through a liquid phase. In centrifugation, materials are separated from one another based on mass, density, and shape. In distillation, liquids are separated from one another, as the one with the lower boiling point will become a gas first and can be collected.

Gas Chromatography

A volatile liquid solution is added to an inert gas carrier like helium or nitrogen, which transports the liquid through a stationary phase. The components travel through the column at different rates, which allows the individual components of the solution to be identified.

High-Pressure Liquid Chromatography (HPLC)

Similar to gas chromatography, in HPLC a liquid sample is injected into the chamber, and pressure is applied to promote the separation of the individual components.

Electrophoresis

Electrophoresis is used to separate macromolecules via an electric field and movement to cathode or anode, depending on how charged the molecules are. Agarose gel electrophoresis separates molecules based on size. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) separates macromolecules based on mass with no impact of charge.

Stereochemistry

Stereochemistry is the study of how atoms are arranged within a molecule and how this structure impacts the molecule’s interaction with its environment. Isomers are molecules with the same molecular formula but different three-dimensional structures. Chirality describes a superimposable versus non-superimposable image. Stereoisomers are molecules with the same composition of elements but with differing spatial organization, such as enantiomers and diastereomers.

Chirality

An achiral molecule is one with a plane of symmetry, whereas a chiral molecule is one with four different groups bonded to the central atom with no plane of symmetry.

Isomer Relationships

Constitutional isomers are ones with the same individual element components, just in a different order, as pictured below:

Retrieved from: https://openstax.org/books/organic-chemistry/pages/5-9-a-review-of-isomerism

Stereoisomers are elements that are connected in the same order but organized differently spatially, as pictured below:

Retrieved from: https://openstax.org/books/organic-chemistry/pages/5-9-a-review-of-isomerism

Conformations

In stereochemistry, a molecule will adapt to the most energy efficient conformation, meaning it will rotate around its single bonds to minimize electron repulsion of the adjacent negatively charged electron clouds. The conformation adapted by a molecule will impact its reactivity and its physical and chemical properties.

Nomenclature

This section covers the rules surrounding naming organic molecules and compounds. The simplest molecules are the alkanes, which are named using the Greek root for the number of carbons and an -ane ending. Everything builds from there. There are many building blocks to consider and rules to follow, detailed below.

International Union of Pure and Applied Chemistry (IUPAC) Rules

The first IUPAC rule of naming is to find the longest chain and name it as the backbone of the molecule. If two chains are equivalent, name the most functional groups attached to it. The next rule is to name the substituents with their appropriate prefix and the -yl ending. These groups are numbered so as to minimize the numbers assigned to the groups. Lastly, list the substituents in alphabetical order with the assigned numbers. Prefixes are ignored when alphabetizing, and commas are placed between numbers.

Functional Groups in Molecules

If the carbons are arranged in a ring, they are named using the cyclo- prefix. Alkenes, or carbon-carbon double bonds, are named similarly to alkanes except the -ene suffix is used. Triple bonds are called alkynes and have the -yne suffix. Alcohols are named by replacing the -e ending with -ol, adding numerical additions for more than one alcohol group like -diol for two.

Ethers are named as alkoxy-, with the alk- component corresponding to the Greek root number prefix. For example, an ether with a one carbon group would contain the prefix methoxy-, and so on. Aldehydes are named with the -al suffix, and ketones are named with the -one suffix. Carboxylic acids are named with the -oic acid ending in place of the -e of the corresponding alkane. Amines replace the final -e with -amine or add the amino- prefix to more complex molecules.