Molecular Structure Flashcards Preview

MCAT Organic Chemistry > Molecular Structure > Flashcards

Flashcards in Molecular Structure Deck (95):

How many valence e- does boron have?



Define a covalent bond.

When electrons are shared evenly between two atoms.


Define ionic bond.

When electrons are transferred from one atom to another, the bond is said to be ionic.


Electronegativity differences that constitute an ionic, covalent, or polar covalent ( partially ionic(

2.0 for ionic; less than 1.5 for covalent; between 1.5 and 2.0 for polar-covalent


Define polar covalent bonds.

Polar covalent bonding is a type of chemical bond where a pair of electrons is unequally shared between two atoms. Like WATER.


Difference between sigma and pi bonds.

Sigma bonds are defined as having e- density shared between the nuclei of the two atoms; pi bonds are defined as having no e- density shared between the nuclei of the two atoms, but instead only above and below the internuclear regions.


What kind of orbitals are sigma bonds made of? What about pi bonds?

Sigma bonds are made from many types of atomic orbitals ( including hybridized). Pi bonds are made exclusively of parallel p- orbitals.


With sigma bonds, most of the e- density is....

Between the nuclei of two atoms.


Where are pi bonds found?

Pi bonds are found as the second bond present in double bonds and the second and third bonds present in triple bonds.


The first type of bond to form between atoms is usually sigma or pi?

Sigma. Exception is F2.


With molecular orbitals, the shading of the lobes in each orbital represents _____?

The direction of spin for the electron.


Order the relative strength of the following bonds: triple, double, single.

In descending order of relative strengths: Triple bond > double bond > single bond


Longer bonds are ( weaker/stronger)?

For bonds between like atoms, the longer the bond, the less e- density overlaps between nuclei, and thus the weaker the bond.


How many sigma and pi bonds are in single, double, and triple bonds?

Single bonds = only one sigma bond. Double bond = one sigma bond and one pi. Triple bonds = one sigma bond and two pi bonds between two adjacent atoms.


HONC 1234, What does this acronym stand for?

In a neutral compound, hydrogen makes 1 bond, oxygen makes 2 bonds, nitrogen makes 3, and carbon makes 4.


True or false? In general, having too many bonds in a molecule results in a cation and two few bonds results in an anion, except with carbon.



Explain hybridization and why it occurs.

Hybridization entails relocating e- density in atomic orbitals prior to bonding, in order to minimize the repulsion between e- pairs and thereby allow for bonding between atoms.
Web definition: The concept of mixing atomic orbitals into new hybrid orbitals (suitable for the pairing of electrons to form chemical bonds in valence bond theory.


sp, sp2, and sp3. What are the associated angles?

180 (linear); 120 (planar); 109.5 (tetrahedral)


True or false? Hybridization explains why e- pairs in valence shells of bonding atoms assume orientations as far from one another as possible. Also explains bond lengths and angles.



*True or false? Hybridization can influence the reactivity and acidity of a compound.



*True or false? The more s-character in the hybrid orbital of the atom bonded to hydrogen, the stronger the acid.

True. Relative acidity sp > sp2 > sp3.


True or false. Electron pairs have no influence on bonding angles.

False. Consider the water molecule. Because there are two pairs of e- on the oxygen, the e- density repels the e- in the two carbon-hydrogen bonds. This forces the bonds closer together, which compresses HOH bond angle.


Describe homolytic cleavage.

The breaking of a chemical bond into two free radical fragments.


*Where does homolytic cleavage typically occurs?

This is typically seen in the gas phase or an aprotic, nonpolar solvent, where ions are too unstable to exist.


Formation of bonds (releases/absorbs) energy.

Energy is released when a bond is formed.


*Breaking bonds (releases/absorbs) energy.

Energy must be absorbed by the molecule to break a bond. An endothermic reaction.


How do you calculate the enthalpy of a reaction?

Energy bonds broken - Energy bonds formed.


Ionic bonds are most common between what kind of elements?

Metals and nonmetals.


The (greater/smaller) charge on the ion, the (weaker/ stronger) the bond. The (closer/further) the ions are to one another, the (stronger/weaker) the bond.

greater, weaker; stronger, stronger


Name the five excuses in organic chemistry.

Resonance, inductive effect, steric interactions, aromaticity, hybridization


Describe what intramolecular forces are.

Intramolecular forces encompasses anything that affects the stability of a molecule and the sharing of e- density beyond the localized region between two neighboring, bonding atoms.


Resonance is an intramolecular phenomemon whereby e- density is shifted through regions of the molecules via (sigma/pi) bonds.

Pi bonds. ONLY pi bonds.


Resonance structures have three priorities. What are they?

1) octets are filled ( excluding H)
2) the best structure minimizes # of formal charges
3) If a molecule contains a - or + charge, it is best placed on the appropriate electronegative/positive atom.


When an amide is placed in an acidic solution, the (O/N) gets protonated because it is more ____.

O gets protonated because it's more electron rich and thus the most nucleophilic site.


Eletronwithdrawing groups (increases/decreases) acidity and (increases, decreases) the pka.

Increases, lowers


*The most basic nitrogen atom is the one (most/least) capable of sharing its lone pair with a proton or electrophile.

Most. This means that the N where the lone pair is least shared within the molecule is the most basic.


What's the acronym for relative EN for common atoms?

F>O>N>Cl>Br>I>S>C>H ; useful in analyzing inductive effect.


The withdrawal of e- density by e- withdrawing groups (increases/reduces) the nucleophilicity of an amine compound by pulling e- away from the N atom.

Reduces. As the e- density is removed, the compound becomes e- poor and thus a worse e- donor.


Caboxylic acids are (stronger/weaker) acids than alcohols.

COOH are stronger acids than alcohols, due to the withdrawing of e- density by the carbonyl oxygen through resonance. Via the inductive effect, can increase acidity.


Describe the inductive effect.

The skewing of e- induced by EN atoms.


True or false? The inductive effect involves only e- donation.

False. You must consider whether the inductive effect involves e- donation or withdrawal.


Which one is more nucleophilic? Methyl amine or ammonia (NH3)?

Methyl amine. The methyl group is also e- donating.


Which is one is more nucleophilic? Trimethyl amine or dimethyl amine?

Dimethyl amine. e- donation by the additional methyl group does not compensate for the increased molecular size (bulkiness).


Describe what steric hinderance is.

Steric hinderance occurs at any time two atoms attempt to be in the same place at the same time.


Substituents on cyclic molecules with (axial/equitorial) orientation experience greater steric hinderance.

Axial orientation.


The greater the forces, the higher the boiling point.

True dat. Memorize this, fool.


Rank the intermolecular forces in terms of strength: hydrogen, Van der Waals, polar interactions ( dipole - dipole)

Hydrogen > polar interaactions > VDW


Describe hydrogen bonding.

A hydrogen has a substantial partial positive charge when it is bonded to a small EN atom such as N, O, or F. Hydrogen bonding involves the sharing of a lone pair of a nearby atom with an electropositive hydrogen.


Describe dipole-dipole interactions.

Coulombic attractions between partially charged particles. Two molecules with dipole moments can attract one another.


Describe Van der Waals forces.

VDW forces exists between all compounds. They are the result of temporary dipoles and are the weakest IM force.


IM forces, ____, and molecular rigidity must be considered when determining boiling and melting points.

Molecular mass.


What is a protic solvent?

The term protic implies that a hydrogen capable of hydrogen bonding is also slightly acidic.


True or false? Hydrogen bonding in alcohols is stronger than in amines.



The (greater/less) number of pi bonds in a compound, the (lower/higher) the melting point.

Greater, lower. Think unsaturated compounds and double bonds.


Define solubility.

The ability of a solute (solid) to dissolve into solution.


Define miscibility.

The ability of a liquid to mix (dissolve) into another liquid.


Remember that like dissolves like.



Name three polar protic examples.

Water, alcohols, amines.


Name three polar aprotic examples.

Ketones, ethers, alkyl halides (methyl chloride)


Give an example of nonpolar, aprotic compounds.

oils, lipids, petroleum.


What IM forces are at play in polar protic solutions?

H-bonding, D-D, VDW


What IM forces are at play in polar aprotic solutions?



What IM forces are at play in nonplolar aprotic solutions?

only Van der Waals.


True or false? In general, the less stable reactant yields a greater amount of heat upon reaction, so the pi bond must be the weakest in an unsubstituted alkene.



Degrees of unsaturation.

One degree of unsaturation indicates the presence of one pi bond or one rings; two DOU means one pi bond and one ring or two rings.

DOU = (2n + 2) -x/2
n = # carbosn
x = # hydrogen or monovalent atoms (F, Cl, Br, I)
When there's a N, replace it by 1C and 1H
Oxygen is ignored.


What is the hybridization of carbocations?

sp2 with an empty p orbital.


Carbocation and carbanion 1, 2,3 carbon stability, reactivity, energy.

More stable/less reactive/lower energy:
3---> 1 = carbocation
1----> 3Carbanios


Organic intermediates are stabilized two ways. What are they?

1) Induction: Electron donating groups (all alkyl substituents) stabilize carbocations; e- withdrawing groups stabilize e- rich intermediates, carbanions. Works through pi bonds.
2) Resonance occurs in conjugated pi systems - containing three or more atoms that each bear a p orbital.


Resonance structure priority rules.

1) Octects satisfied.
2) Resonance minimizes separation of charge (formal charge) are better than those with large separation of charge.
3) In structures with formal charges, the most important resonance contributor has negative charges on the more EN atom(s), and a positive charge on the less EN atom(s).


What should we look for when attesting stability of an acid.

To rank the relative acidity of organic compounds, consider which conjugate base is the most stable.


For alkoxides, why aren't there any resonance structures when an H is removed from the hydroxyl.


The carbon has no unhybridized p orbital. Therefore no resonance delocalization of the adjacent lone pairs is possible.


Rank acidity for the following: sulfonic acids, phenols, carboxylic, alcohols and water, aldehydes and ketones, sp1 - sp^3 hybridized C-H bonds

Strong acids (HI, HCl, HNO3, H2SO4) > sulfonic aids, carboxylic acids> phenols> EtOH and H2O> aldehydes and ketons> sp hybridized CH bonds, sp2 hybridized CH bonds, sp3 hybridzed C-H bonsd.


Which is the stronger acid: triflouroacetic acid (pka 0.23) or tribromoacetic(pka = 0.73) acid.

Triflourocetic acid. Lower pka = stronger acid.


Comment on nucleophilicity in terms of negative charge, going down a periodic group, and across a particular period.

1) Increases as negative charge increases (-NH2 > NH3)
2) Increases going down the periodic table within a group (I>Br>Cl>f)
3) Increases going left in the periodic table acoss a particular period. NH2- >OH-


Nucleophiles versus

Nucleophile: species that have unshared pairs of e- or pi bonds, frequently, a negative charge (or partial negative) charge. Nucleus seeking; e- donors so lewis base.
Electrophile: electron deficient species; partial positive charge; e- pair acceptors so also known as Lewis Acids.


What makes a good leaving group.

1) resonance stabilized leaving groups.
2) weak bases (I->Br->Cl-) are good leaving groups because of their size


Do strong bases make good leaving groups?

No, they are great e- donors because they cannot stabilize their negative charge very well, making them really reactive.


How can you make an alcohol likely to lose its hydroxy?

You can add an acid so it protonates the hydroxyl group.


Can you hydrogenate rings to turn them into its akyl form?

Only for cyclopropane and cyclobutan, since these are high energy molecules due to ring straing.

Cyclopropane --> H2 + Ni + heat ---> propane


Conformational isomers.

Compounds that have the same molecular formula and the same atomic connectivity, but differ from one another by rotation about a sigma bond. Staggered versus eclipsed is an example. An


Constitutional isomers.

Compounds that have the same molecular formula but have their atoms connected together differently.


For 1,2 ethandiol, why is the gauche conformation more stable than anti.

In intramolecular hydrogen bond can be formed in the gauche.


Why are 1,3 diaxial interactions not favorable?

It's more favorable for certain moelcules to be in the equatorial position to avoid sterics.



Molecules that have the same MF AND connectivity but differ from one another only in the spatial arrangement of the atoms.


Define enantiomers and give properties.

Most chemical properties (BP, MP, polarity, solubility) are the same for both pure enantiomers; they have identical physical properties. The only difference appears to be the manner in which they interact with plane-polarized light, which is equal in magnitude but in opp directions. They are chiral and they interact differently with other chiral molecules but not with other achiral molecules.


Optically active.

A compound that rotates the plane of polarized light. These would be chiral molecules.


What's the specific rotation of an equimolar mixture of two enantiomers?

The rotation should be 0 degrees. THis is a racemic mixture.


True or false.

+ and - say nothing about whether the absolute configuration is R or S. There's no correlation between the sign of rotation and the absolute configuration.



Stereoisomers that are not enantiomers.


How does one typically synthesize an enantiomer?

The racemic mixture is made and then a process called "resolution", where a chiral probe (aka a resolving agent) is used to associate with a particular enantiomer, creating diastereomers. This changes the molecule's physical properties. Filtration proceeds and then finally removal of the probe.



A subclass of diastereomers that differ in their absolute configuration at a single chiral center (only ONE stereocenter is inverted).



Epimers that form as a result of ring closures are known as anomers. C1 orientation distinguishes the alpha and beta anomers in sugars.


Meso compounds.

When there's a plan of symmetry in a molecule that contains chiral centers ( at least 2).


Are meso compounds optically active?

They are not. They have chiral centers but aren't optically active.


Geometric isomers.

Diastereomers that differ in the orientation of substituents around a ring or a double bond.