Introduction to Organic Chemistry

A Lewis structure, or Lewis electron dot formula, gives information about each atom's valence electrons. What are the three rules to consider when drawing Lewis structures

Exceptions (to drawing Lewis structures): On occasion the atoms within a molecule will break the octet rule. The atoms may have less than an octet or more than an octet. _____ and ______ do not contain full octets. Atoms from the _____ period or higher in the periodic table may be able to hold more than 8 valence electrons due to vacant ___ _______ available for hybridization, as discussed in Section 2.4. _____ is the element most commonly seen with an expanded octet.

There is no need to consider which electrons come from which atoms in Lewis structures. Simply count the total number of valence electrons and distribute them to complete the valence shells. Knowing each atom's valence (define) can be helpful. Some valences for common atoms in organic chemistry are as follows: carbon is _____ (4 valence electrons); nitrogen is _____; oxygen is _____; hydrogen and the halogens are ______ (though halogens other than fluorine can be ______ on rare occasions).

What are the common valence representation for: 
Hydrogen, Carbon, Nitrogen, Oxygen, Flourine, Sulfur, Phosphorus, Silicon and Chlorine

A Lewis structure can be used to determine the formal charge (state the formula) of an atom. The formal charge is the number of _____ _____ of an atom, minus the number of _____ it is a part of, minus the number of ______ _____ it has. *When counting bonds, double bonds count as two bonds and triple bonds count as three.

In a cyanide ion, carbon has _____ bonds and ____ nonbonding electrons for a total of ____ electrons. A neutral carbon atom has only _____ valence electrons, so the formal charge on carbon in the cyanide ion is ____ ____.

The sum of the formal charges for each atom in a molecule or ion represents the ____ charge on that molecule or ion, but the formal charge, on a given atom does not represent an actual _____ on that atom. Determining the true charge distribution requires consideration of _______ differences between all the atoms in the molecule.

There are many ways to represent molecules other than the Lewis structure. The _____ formula shows the bonds between each atom of a molecule, but does not usually display lone pairs. The ____ formula also does not show the three dimensional structure of the molecule. By contrast, the ______ formula shows neither the bonds nor the three-dimensional structure. Central atoms are usually followed by the atoms that bond to them even when this is not the bonding order. For instance, the three hydrogens following the carbon in CH₃NH₂ do not bond to the nitrogen.

The _____ _____ formula is the most prevalent representation of molecules in organic chemistry questions on the MCAT®. In the bond-line formula, line intersections, corners, and endings represent a _____ atom unless a different atom is drawn in. The ______ atoms that are attached to carbons are not usually drawn; it is assumed that the correct number are present to give each carbon _____ bonds. Do not forget about these hydrogens even though they are not shown. Bond-line diagrams are an easy way of representing large molecules.

Fischer projections are also common on the MCAT®. In Fischer projections ____ lines are assumed to be oriented into the page. ______ lines are assumed to be oriented out of the page. Fischer projections are often used to represent _______ and are an easy way to give information about the three dimensional shape of a molecule.

The Newman projection is a view straight down the axis of one of the σ-bonds (discussed later in this lecture). Both the intersecting lines and the large circle represent ______ atoms. Newman projections give information about _____ _______ with respect to a particular σ bond.

In the dash-line-wedge formula , the ____ ____ wedges represent bonds coming out of the page, the ______ wedges represent bonds going into the page, and lines represent bonds in the plane of the page.

The space-filling model is a 3D representation of a molecule, with spheres of various ______ representing different elements with respect to their relative _____.

The atomic radii for atoms that are part of ball and stick models in this manual are drawn to scale. However, bond lengths are drawn to approximately ____ their length so that the atoms are clearly visible. Ball and stick representations give information about the relative _____ of atoms.

Pi bonds are more reactive than _____ bonds. _____, _____, _____, and _____ are the atoms that most commonly form π bonds. Phosphorous forms ____ bonds with oxygen in nucleotide phosphates such as ATP.

Atoms may form multiple bonds, depending on the number of _____ _____ they have available. Carbon is an example of a molecule that can form ____ bonds with its ____ valence electrons. These bonds may be single bonds, double bonds, and/or triple bonds. Carbon forms four _____ bonds in CH4; a _____ bond (and two single bonds) in C2H4; and a _____ bond (and one single bond) in C2H2 .

The electrons in a σ bond (define) are as _____ as possible to the two sources of positive charge (the two _____). Therefore, a σ bond is the _____ energy, _____, ____ stable type of covalent bond.

A σ bond is always the _____ type of covalent bond to be formed between two atoms, so a _____ bond must be a σ bond. Double bonds and triple bonds also contain ____ σ bond each.

A π bond (pi bond) is created by ______ p orbitals. Double and triple bonds are made by adding π bonds to a σ bond. The σ bond leaves no room for other ______ ______ directly between the atoms, so the first π bond forms ______ and _____ the σ bonding electrons, forming a _____ bond between the two atoms.

A double bond always consists of ____ π bond and ____ σ bond. If another π bond is formed, the new orbital is formed on either side of the ___ bond, forming a ____ bond between the two atoms. Triple bonds are always made of ____ σ bond and ____ π bonds.

Bond energy is the energy needed to _____ a bond). A π bond itself is _____ than a σ bond (less energy is required to break the bond), but π bonds are always added to an existing ____ bond, and thus ______ the overall bond between the atoms. Therefore the overall bond energy is ______ as π bonds are added.

At the same time, adding a pi bond ______ the overall bond length, since bond strength is ______ related to bond length. A double bond is therefore _____ than a single bond. Overall, single bonds are the longest and _____ to break; double bonds are shorter and _____ to break; and triple bonds are the shortest and _____ difficult to break.

Atoms that are bound by a single bond can ______ freely around the bond, changing the overall ____ of the molecule. However, when π bonds are present, free ______ is no longer possible. ______ bonds essentially "lock" the molecule into place, introducing rigidity in molecular structure, as will be further discussed later in this lecture.

You know that a π bond is more reactive than a σ bond. Why, then, are double bonds harder to break than single bonds?

The four valence electrons of a lone carbon atom in its ground state are in their expected atomic orbitals: ____ in the orbital of the 5-subshell and ____ in orbitals of the p-subshell. The p electrons are at a _____ energy state than the 5 electrons. What are the atomic orbitals of a lone carbon atom?

However, the bonds of a carbon with four σ bonds are typically ______ to one another. Since the bonds are indistinguishable, the ______ which form them must be equivalent. Use the theory of hybrid orbitals to explain how this is possible.

A σ bond is formed in the area where the _____ _____ of two atoms overlap. π bonds are formed by the overlap of _____ ___ _____.
Label this figure:

Hybrid orbitals are named according to the type and number of orbitals that overlap to create the hybrid orbital: sp, sp2, sp3, dsp3, d2sp3, etc. How do you determine the number and type of hybrid orbital formed by an atom?

The MCAT® is unlikely to test hybridization of d and f orbitals, which can occur only after all ____ s and p orbitals have hybridized. The oxygen atom in H2O makes ____ sigma bonds and has ____ lone pairs of electrons. Since the number of hybrid orbitals must equal the number of _____ _____ and _____ _____, the oxygen atom must have _____ hybrid orbitals, or ____ hybridization.

In terms of shape and energy, a hybrid orbital resembles the __ and __ orbitals from which it was formed to the same extent that these orbitals contributed to its formation. This is referred to as character. The superscripts indicate the character: an sp2 orbital is formed from ___ s and ___ p orbitals and thus has ____% s character and ____% p character; an sp orbital is formed from ____ s- and ____ p-orbital and thus has ____% s character and ____% p character; and so on. The more s character a bond has, the _____, _____, and more _____ it is.

Explain the Valence Shell Electron Pair Repulsion (VSEPR) theory
State the bond angles and shape of the following hybridizations

The specific shape formed by an atom's hybrid orbitals depends upon the number and position of _____ _____. _____ ____ and ___ electrons require more room than bonding pairs, which means they can distort the predicted bond _____ (as can ring strain). The lone pairs on water make the bond angle 104.5°, rather than the expected angle of 109.5°.

Sometimes bonding electrons are spread out over three or more atoms. These electrons are called ______ electrons. For the purposes of the MCAT, delocalized electrons only result from ______ and _____ _____.

Molecules containing delocalized electrons can be represented by a combination of two or more alternative Lewis structures called ______ ______. The weighted average of these Lewis structures most accurately represents the actual molecule. The real molecule exists at a _____ energy than any single Lewis structure that contributes to it. Otherwise it would simply exist as that ______ ______. The difference between the energy of the real molecule and the energy of the most stable Lewis structure is called the ______ energy.

Three rules for drawing resonance structures

The contribution made to the actual molecule by any given structure is roughly proportional to that structure's _____. The most _____ structure makes the greatest contribution, while equivalent structures make ____ contributions. In general, the _____ the formal charges on most atoms, the more stable the structure. Separation of charges within a molecule ______ stability.

Two conditions are required for resonance to occur:

The adjacent p-orbital in a conjugated system may contain ____, ____ or ____ electrons (as in another n bond). The p-orbital allows the adjacent n bond from the double or triple bond to extend and encompass more than ____ nuclei.

Define aromaticity
State Huckel's rule

When considering molecules with delocalized electrons, the _____ moment provides information about which side of the molecule has greater electron density. Electrons concentrate around the area of highest _______, giving the area partial _____ _____ character, labeled δ-. The less electronegative side has a partial ______ ______ character, labeled δ+.

Electronegativity, or how much an atom "likes" electrons, provides information about a molecule's charge distribution  (explain). Electrons that are shared between atoms that differ significantly in electronegativity are drawn toward the ____ electronegative atom.

To determine the charge distribution of an unfamiliar molecule, first determine whether it has a _____. Use trends in ______ to determine which areas are centers of positive charge and which areas are centers of negative charge. Knowing how the ____ is distributed in a molecule allows for the prediction of how it will behave in reactions. These functional groups are groups of atoms on a molecule that are involved in ______ and behave in _______ ways. They are _____, non-_____ portions of molecules.

The first step in solving any organic chemistry problem is to recognize which _____ _____ are involved in the reaction. The MCAT® tests only the reactions of common _____ _____. Many molecules on the MCAT® will appear large and unfamiliar, but answering questions about them only requires familiarity with the attached functional groups and how they react.

A molecule may have a complicated structure, but its chemistry is governed by its functional groups. For example:

Phenol and ethanol undergo many of the same reactions even though they have different structures (why?)

Functional groups can be divided into two main groups according to their overall electronic character:

Focus on the _____ of the functional group, rather than the details of each reaction. A general knowledge of functional groups and their charges makes it possible to predict the outcomes of unfamiliar reactions.

Below are two lists of functional groups that are commonly seen on the MCAT . Examine each group and try to determine how it might behave in a reaction.