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UCD / Biology / BIOL 8 / If we have a chiral center, how do we know where it points?

If we have a chiral center, how do we know where it points?

If we have a chiral center, how do we know where it points?

Description

School: University of California - Davis
Department: Biology
Course: Organic Chemistry - Brief
Professor: Sarah lievens
Term: Summer 2015
Tags: organic and Chemistry
Cost: 50
Name: CHE 8A: Quiz 5/9 Study Guide
Description: This Study Guide Compiles the important sections of the past 2 weeks notes for the quiz
Uploaded: 05/06/2016
7 Pages 47 Views 1 Unlocks
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CHE 8A: Quiz 5/9 Study Guide


If we have a chiral center, how do we know where it points?



● Cycloalkanes​→ longest chain = ring

○ Number so you get the lowest possible #’s on substituents

○ If a tie → go ABC order

● 1 substituent: 

○ Methyl cyclohexane → only 1 substituent, has to be C #1


What are the different kinds of stereoisomers?



○ Writing 1-methyl… would be unnecessary

● 2 substituents: 

○ 1 substituent will be #1 and the other as low as possible If you want to learn more check out What country are the new sources of aid?

○ TIE: so we go with ABC order

■ * 1-chloro-3-ethylcyclopentane *

■ X 3-ethyl-1-chlorocyclopentane X

● 3 (or more) substituents: 


How a molecule interacts with a different substance?



○ Aim for lowest possible numbers

○ If 2 or more are on the same C, ALWAYS make that C #1

○ 3-tert-butyl-1-fluoro-1-methylcycloheptane

● Boiling Points:

○ Measure of energy it takes to break up all intermolecular forces ■ Not the molecules themselves If you want to learn more check out What is learning & memory in behavioral neuroscience?

○ Higher boiling point (BP) = more intermolecular forces

■ More “sticky” to itself

● Alkanes: only one kind of Intermolecular Force (IMF)

○ London Forces / Dispersion Forces

■ Temporary dipoles; very weak

■ Move around like at a party, leaving an average overall force

○ London Forces: ​random motion of e- clouds → create temporary dipole ■ Increase with larger molecules = more stuff = more e-

■ ↑mass ↑BP If you want to learn more check out What is the meaning of intrinsic in motivation?

■ Increase with greater Surface Area (more dipole to contact)

■ ↑Surface Area ↑BP

● Haloalkanes: increased BP when compared to Alkanes If you want to learn more check out What is the popperian view of science?

○ Permanent polar bonds / permanent dipole

○ Bigger orbitals = squishier = polarizable

○ Adding a non-C element → ads a permanent dipole We also discuss several other topics like What is the dispersion force that causes them?

■ Increases BP: Stronger IMF = dipole/dipole​, polar interactions ○ ↑Dipole ↑BP

● Hydrogen Bonds: strongest IMF

○ Between H and O,N, or F (with lone pair on O,N,F)

○ Huge dipole moment bonding to H

■ H2O → 2 H-Bond per O → BP 100°C

■ NH3 → 1 H-Bond per O → BP -33°C

■ CH4 → 0 H-Bond per O → BP -167.7°C

● Solubility:

○ How a molecule interacts with a different substance

○ How 2 substances dissolve together

○ Like dissolve Like We also discuss several other topics like What approach assumes members are rational?

■ Different IMF = less soluble

○ Highly polar (H2O): can dissolve Polar things

■ Amines, alcohols, carboxylic acids, ketone esters

○ Non-Polar (alkane, oil): can dissolve non-polar

■ Alkanes, haloalkanes, alkenes, alkynes

○ Slightly Polar (ether): can dissolve most stuff at least a bit

■ Ethers, ketones, esters, haloalkanes, amine, alcohols

○ Ions: dissolve in H2O

● Into 3 Dimensions:

○ 2 extreme structures

○ (solid triangle = to us, dashed = away from us)

○ Staggered Rotamer:

■ Essentially looking at angle from the side

■ Angle btwn H’s on 2 different C’s = 60 degrees = torsional angle ■ Center = C1, Ring = C2

○ Eclipsed Conformer:

■ All C-H bonds lineup

■ H’s directly eclipsed by one another

■ 0 degrees torsional angle

○ Skew Angle = a​nything else (any other torsional angle)

● Cycloalkanes formula:

○ CnH2n → all C+H no ends

● Ring Strain:​not all rings are the same

○ Energy difference btxn cyclic + linear structure

○ 2 aspects:

■ 1. Bond angle

● Wants to be tetrahedral (109.5 degrees)

■ 2. Eclipsing strain

● Wants to be staggered (60 degrees)

● 4 different rings:

○ Propane:

■ 1. Has 60 degrees =/= 109.5

■ 2. 100% eclipsed

■ Lots of strain ~30 kcal

○ Butane:

■ 1. Has 90 degrees

■ 2. Avoids 100% eclipsing by “puckering”

● Not flat or straight anymore

■ Lots of strain ~28kcal

○ Pentane:

■ 1. Has 108 degrees ~~ 109.5

■ 2. w/ planar: have 10 eclipsing C-H

● Also exists as puckered (increases strain)

■ Minimal strain ~6 kcal

○ Cyclohexane:

■ When flat (planar) looks awful

■ 1. Has 120 degrees ~~ 109.5

■ 2. 12 eclipsing C-H (lots of strain)

■ → puckers into a comfortable chair position

○ Chair: zero ring strain

■ 109.5 degrees, 100% staggered

■ (larger molecules have some ring strain

● Cyclohexanes:

○ 3 sets of parallel lines

○ Each C has 2 H’s in different relative positions

○ Axial H - parallel to axis

■ Straight up/down with the bend of the point

○ Equatorial H - point out around equator

■ Parallel to the side it doesn’t touch

○ Each C has one equatorial and one axial H (one up and one down)

● If substituents aren’t just an H - different energy levels

○ Prefer to be equatorial

○ If more than one or a tie, the larger goes on the equatorial

○ Isomers → same formula / different molecules / different properties ● Constitutional Isomers:

○ Different bonds / different names

● Stereoisomers:

○ Same bonds / different orientations

○ Different shapes / different properties

○ Different prefixes but same overall name (cis/trans/R vs S/E vs Z) ○ Different Kinds of Stereoisomers: 

■ Diastereomers:

● Same bonds / not superimposable / not same molecules

● Not exact mirror images

● Totally different properties (fraternal twins)

■ Enantiomers: (relationship between 2 molecules)

● Same bonds / not superimposable / not same molecule

● Exact mirror images (left vs. right hand)

● Nearly identical properties (BP / Solubility)

● Vary in optical rotation with other Chiral molecules

● Chiral: (inherent property)

○ Have enantiomers but are overall asymmetrical

● Chiral Center:

○ Need asymmetry / cannot rotate to produce a mirror image

○ Takes 4 different groups on the same sp3 Carbon to be Chiral

■ Can’t match or superimpose

■ Molecules themselves can be chiral

● But can also be enantiomers

● (molecule is asymmetrical but it also has a reverse twin)

○ A molecule can be overall achiral but still have chiral centers

● If we have a chiral center, how do we know where it points?

○ Assign R vs. S

○ 1. Assign priority to each of 4 groups around C

■ Highest atomic #

■ Double bonds count as 2 of that molecule, triple as 3

○ 2. Lowest priority in the back (always H)

○ R = clockwise = right turn

○ S = counterclockwise = left turn = sinister

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