lecture 2 recap

Question 1

the b.p of a compound depends on

Answer 1

the strength of the attractive forces between the individual bonds

Question 2

what are London dispersion forces and how do they occur

Answer 2

they are induced-dipole-induced dipole interactions that occur in nonpolar molecules such as alkanes, where the overall average charge is neutral, however, due to the continuous movement of electrons through the compound at one instant the electron density might be greater on one side which then causes a temporary dipole that dipole would then disrupt the electron density of its neighboring molecules where the temporarily negative side of a molecule ends up adjacent to the temporarily positive side of the of the neighboring compound causing them to have an induced dipole forming the London dispersion forces

Question 3

what is the r.s between London dispersion forces (in alkanes) and molecular weight of a compound

Answer 3

as the molecular weight of an unbranched organic compound increases the London dispersion forces increase as there is more contact area with the added CH2

Question 4

what is the r.s between London dispersion forces (in alkanes) and and the branching of a compound

Answer 4

as the branching in a compound increases the strength of the London dispersion forces decreases as there is a lower contact area so if two compounds have the same Mr then the one with the least branches has a higher B.P

Question 5

what is dipole-dipole interactions

Answer 5

it is a strong electrostatic force between two polar atoms where they always align themselves to have the positive ned of the dipole adjacent to then negative end of the dipole

Question 6

what is polarizability mean, and how does it increase

Answer 6

polarizability indicates how readily an electron could be disrupted to form a strong induced dipole and it increases as the anion increases in size and charge (check)

Question 7

what is a hydrogen bond and why does it increase the B.p of a compound significantly

Answer 7

it is a special kind of dipole-dipole interaction between a hydrogen attached to nitrogen, oxygen, or fluorine and a lone pair on oxygen, nitrogen, or fluorine on a different molecule. it is significant since there are multiple hydrogen bonds that can form within a compound overcoming the fact that the bond itself is weak (note: the strongest hydrogen bond is linear 180*)

Question 8

what is the r.s between the B.p and the order of an amine

Answer 8

as the order of an amine decreases the B.p increases as primary amine can form more (and stronger) hydrogen bonds than secondary amines, and secondary amines can form more hydrogen bonds than territory amines since territory amines have no hydrogen attached to the nitrogen

Question 9

what are the two things that M.p is affected by

Answer 9

1. Packing in the metal lattice the tighter the fit the more energy is required to break the lattice
2. intermolecuelar forces)

Question 10

what is the difference between odd-number alkanes and even-number alkanes when it comes to M.p

Answer 10

when reviewing their melting pattern each forms a smooth curve with no overlap, but we notice that the odd number alkanes have a lower M.p and that is because they fit less tightly together due to their structure they always lie next to each other with a methyl group on the end repealing the other methyl group on the end of the other molecule thus increases the distance between them and making them have weaker intermolecular bonds

Question 11

what is the general rule that governs solubility?

Answer 11

polar substances dissolve in polar solvents and non-polar substances dissolve in non-polar solvents

Question 12

what is solvation

Answer 12

it is the interaction between solvent molecules and solute molecules where the negative poles of the solvent surround the positive poles of the compound and vice versa causing this clustering that operates the compound molecules

Question 13

the r.s between an alcohol’s solubility and its branching

Answer 13

as the branching of an alcohol increases the more soluble it is due to less area contact with the hydrogen bond

Question 14

what is the 4-carbon alcohol divide line in solubility

Answer 14

any unbranched carbon passed 4 is hard to dissolve in a polar solvent such as water

Question 15

What is the 3-carbon ether divide line in solubility

Answer 15

any unbranched ether molecules with more than 3 carbons are very hard to dissolve in polar solvents such as water

Question 16

what is the most soluble alkali halide and why

Answer 16

it is alkyl fluoride since it can format hydron bonds with polar solvent like water

Question 17

what is the trend in solubility for amines

Answer 17

primary amine > secoundary amine > teritry amine. where all are double since the nitrogen has a lone pair to form a hydrogen bond but the primary has two hydrogens to form more hydrogen bonds while the secondary only has one and tertiary has non

Question 18

what are the arrangements for cis and trans isomers in cyclic compound

Answer 18

if they are both on the same side of the compound then they are cis if on different sides then trans

Question 19

why is cis-trans isomers considered different than the rotation that occurs around the C-C bond

Answer 19

because the energy barrier to rotate a compound around the C-C bond is low and requires more reaction but to rotate a compound around the C=C bond requires the breaking of the pi bond and a lot of energy also the compound that forms due to rotation around the C-C have very similar physical and chemical properties while the rotation that occur around C=C bond forms completely different compounds with different B,pa nd even sometimes dipole

Question 20

what are the arrangement of cis-trans isomers in alkenes

Answer 20

cis if the molecules across the C=C are on the same side, and trans if they are on the same side

(note: if two identical molecules are attached to the same carbon it is not a geometric isomer)

Question 21

why was the E-Z system introduced instead of the cis-trans systems for alkenes

Answer 21

to overcome the struggles with naming alkenes that do not have hydrogen attached to each Sp^3 orbital

Question 22

what is the arrangement for Z-E alkene isomers

Answer 22

Z is when both high-priority atoms/groups are on the same side across the double bond

E - is when the high-priority atoms/group and opposite to each other across the double bond

Question 23

what are the rules that determine which atom/group takes priority

Answer 23

1. Compare the substituents attached to the same sp2 carbon atom
2. Look at the first atom
3. The substituent with the higher atom number (e.g. C vs H) has priority
4. If the first atom has the same atomm number, look at the atoms
   attached to this first atom:
   highest atom number or highest amount of high atom number wins
5. Double and triple bonds count for 2 or 3 atoms, respectively
6. If still a tie: move to the next level
7. In case two different isotopes are connected: the higher isotope wins
   (e.g.D vs H)

Question 24

what is a chiral

Answer 24

an object that forms a non-superimposable image

Question 25

what is an asymmetric centre

Answer 25

it is an atom that is bonded to four different atoms/groups (known as a chiral centre)

Question 26

what are enantiomers

Answer 26

they are 2 non-superimposable mirror image molecules that both have a chiral centre

Question 27

what are sterocenters

Answer 27

it is an atom at which the interexchange between the two groups forms a stereoisomer where it has a asymmetric center where the exchange produces enantiomers and it also has a sp^2 carbon in alkene or an sp^3 carbon in a cyclic compound where the exchange would form cis and trans isomers

Question 28

what is Fischer's projection

Answer 28

it represents an asymmetric carbon centre as a point of intersection between two perpendicular lines where: ■ Horizontal lines represent the bonds that project out of the plane of the paper toward the viewer. ■ Vertical lines represent the bonds that extend back from the plane of the paper away from the viewer. ■ The carbon chain is usually drawn vertically, with C-1 at the top. (to form an enantiomer just flip the two atoms/groups on the horizontal axis)

Question 29

what is the Cahn-Ingold-Prelog system

Answer 29

it is a system of nomenclature that uses R, and S to determine which enantiomer we are talking about depending on the arrangements of atoms/groups around the asymmetric carbon (R - clockwise, and S- anti clockwise)

Question 30

how do you name enantiomers using the R,S system (perspective)

Answer 30

1. Rank the groups (or atoms) bonded to the asymmetric center in order of priority. 2. If the group (or atom) with the lowest priority (4) is bonded by a hatched wedge: ■ Draw an arrow in the direction of decreasing priority—(1) to (2) to (3). ■ If the arrow points clockwise, then the compound has the R configuration (R is for rectus, which is Latin for “right”). ■ If the arrow points counterclockwise, then the compound has the S configuration (S is for sinister, which is Latin for “left”). ■ The letter R or S (in parentheses) precedes the systematic name of the compound. 3. If the group (or atom) with the lowest priority (4) is not bonded by a hatched wedge: ■ Interchange group 4 with the group that is bonded by a hatched wedge. ■ Proceed as in step 2—namely, draw an arrow from (1) to (2) to (3).

Answer 31

1. Rank the groups (or atoms) that are bonded to the asymmetric center in order of priority. 2. Draw an arrow from (1) to (2) to (3). If the arrow points clockwise, then the compound has the R configuration; if it points counterclockwise, then the compound has the S configuration, provided that the group with the lowest priority (4) is on a vertical bond. (if the lowest priority atom/group is in the vertical axis then the configuration is switched)

Question 32

are properties of enantiomers similar

Answer 32

yes so most physical properties of enantiomers are the same the only difference they have is just due to how the 4 atoms/groups are arranged in space which causes each enantiomer to have different optical activity

Question 33

what is plane-polarized light

Answer 33

it is light that only oscillates in one plane and it is produced by passing normal light through a polarizer

Question 34

what is an optically active molecule

Answer 34

it is a compound that rotates the plane of plane-polarized light

Question 35

what is the name of the compounds that rotate the plane of polarized light clockwise, and the name of the compound who rotates it anti-clockwise

Answer 35

dextrorotatory - clockwise laevorotatory - anti-clockwise

Question 36

what is a polarimeter and how does it work

Answer 36

it is an instrument that is used to measure the rotation of the plane of polarization of plane-polarized light. it works with the following process, first off it is important to note that the amount of rotation carries with the wavelength of the light used so the light source must be monochromatic (sodium arc is usually used). Monochromatic light passes through the polarized lens and emerges as monochromatic light, then light passes through an empty tube and emerges with no rotation then the light passes through an analyzer which is the second polarized lens mounted with an eye peace and a daily in degrees the observer will then move the analyzer until no light is shown (perpendicular to teh first polarizer) and the analyzer will be at zero rotation. Now the sample that we want to test is put in the tube and if it is optically active it will then rotate the plane of plane-polarized light and the observer will shine light again, then the amount of rotation required to move the analyzer until no light is shown again will be the observed rotation

Question 37

what is observed in rotation

Answer 37

it is the amount that the analyzer is rotated and it depends on: 1. the number of optically active molecules the light encounters so basically the concentration of the sample and the length of the tube 2. teh temperature and the wavelength

Question 38

what is the equation to calculate the specific rotation

Answer 38

check book page 179

Question 39

what is a racemic mixture

Answer 39

it is a mixture with equal amounts of enantiomers present making the solution optically inactive since both enantiomers rotate the plane of polarized light to same amount

Question 40

what is observed specific rotation

Answer 40

the specific rotation for a particular sample

Question 41

if a sample is enantiomerically pure it means that

Answer 41

the sample only has one enantiomer

Question 42

what is enantiomeric excess and how do you calculate it

Answer 42

enantiomeric excess which is also called optical purity measures the amount of excess enantiomer in a sample: ee = ( (observed specific rotation)/(specific rotation of the pure enantiomer) ) x 100%

Question 43

what is the equation to calculate the amount of stereoisomers a compound can form

Answer 43

2^n (where n is the number of asymmetric centers) this is only true when we don't have a stereocenter can form cis-trans isomers (also this is not true for cyclic compounds but i need to check with the teacher)

Question 44

what are erythro enantiomers and threo enantiomers

Answer 44

they are stereoisomers with two asymmetric centres where when drawn with Fischer projections the hydrogens would be on the same side of the carbon chain (erythro) and where the hydrogens are on opposite sides of the carbon chain (threo)

Question 45

what is the biggest issue that occurs when using Fischer projections

Answer 45

Fischer projection dont represent the molecule in a 3-D shape, so whenever we draw molecules they would be represented in their less stable eclipsed form.

Question 46

what are diastereioisomer

Answer 46

it is a stereoisomer that are not enantiomer (meaning they don't have the same chemical and physical properties). They occurs when there is more than one asymmetric centre in a compound (this definition also includes cis-trans isomers

Question 47

stereoisomers of cyclic compounds

Answer 47

since cyclic compounds can form cis-trans isomers we would expect a greater number of stereoisomers to form but sometimes when we have two asymmetric centres within a cyclic compound, a lesser amount of enantiomers form due to symmetry the mirror image reflection about both asymmetric centre might give the same compound

Question 48

what is a meso compound

Answer 48

it is a compound that has an asymmetric center but is achiral since it is superimposable to its mirror image. This compound is optically inactive for example when we have 2 asymmetric centers the plane of symmetry cuts the compound in half with each half being a mirror image of the other so if one symmetric centre rotates the molecules the other rotates it back (note that the staggered conformer of the molecule might not have a plane of symmetry but as long as one of the different ways we can draw a compound has a plane of symmetry then it is considered meso compound)

Question 49

A compound with the same four atoms or groups bonded to two different asymmetric centers will have

Answer 49

three stereoisomers, 1 meso compound, and 2 enantiomers

Question 50

a cyclic compound that has identical substituents will form

Answer 50

1 meso compound which is the cis-compound, and 2 enantiomers the trans and its mirror image

Question 51

the process of separating enantiomers is called

Answer 51

resolution of racemic mixture

Question 52

how can we separate enantiomers apart

Answer 52

1. by hand as some crystals of enantiomers would be left-sided and right-sided 2. by chromatography, as the mixture of enantiomers is dissolved in a solvent and then is passed through a column packed with chiral material (chiral probe) that absorbs organic compound. Each enantiomer would travel through the column with different rates as they both have different affinities to the chiral molecule so we can collect them at separate times.