midterm 3 Flashcards
what is IR spectroscopy?
-electromagnetic energy in the infrared region (source) interacting with your molecule
*your molecule has vibrating bonds (stretches, bends, twists)
*absorption happens when the specific wavelength of light being used matches the FREQUENCY of the particular vibration
*most covalent bonds have will a unique signature for that specific functional group
- wavelength=distance between peaks
- electric field and magnetic field propagate with the wavelength
- low energy waves are used for NMR while higher frequency/energy waves are used for IR
- energy is proportional to inverse wavelength (as we increase wavelength, there is decrease in frequency and energy)
- spectroscopy: expose sample with electromagnetic wave, put in detector, and it reads out what wavelength is absorbed
- when a specific frequency matches the frequency of our source, we have an absorption
- most covalent bonds have a unique peak that helps us identify what specific functional groups are present in the molecule
overview of functional groups
- sp/sp2 CHs are usually greater than 3000 cm^-1
- sp^3 CHs we usually less than 3000 cm^-1
- C-O alcohol in 1000-1300 cm^-1
- symmetrical stretch: both stretch in the same way (straight down or straight up)
- unsymmetrical stretch: one stretches up and one stretches down
- unsymmetrical bend: both swing to one side
- symmetrical bend: bond swing towards each other (also called wags and scissors)
- C—O and O—H stretch are more telling about what functional groups you have
- below 1400 cm^-1 range is called the fingerprint region (lots of different bending modes and C—C stretches so not lots of useful info except C—O stretch being present or not)
- you are looking at light transmitted (or light absorbed) as a result of “matching” between the wavelength of light and frequency of a particular version
change in dipole moment is needed
why?
*propagating light is a wave that carries a propagating electric field with it
*that electric field will interact with “charges” in different functional groups base on dipoles in certain bonds
*without a change in dipole moment (“moving charges”), the light (and electric field) can’t interact with a particular vibrating bond
- in order to observe a signal in IR spectroscopy, we look at absorption of light or transmittance of light
- to have an IR signal we need a change in dipole moment
- change in dipole moment creates partial positive and partial negative charges that build up on the electronegative or electropositive atom (the change in dipole is what interacts with light)
- unsymmetrical alkenes do have a stretch because it has a change in dipole moment during the stretch (even though its small, it’s still present)
- symmetric C=C bond doesn’t have much of a C=C stretch because there is no change in dipole moment (electropositive and electronegative atoms have same distance to each atom even after you try to stretch it
- the point is to look at bonds to see if extending or compressing them will lead to a change in dipole moment
- C—O bonds usually always have a change in dipole moment
- make sure to check if there is a change in dipole after a potential stretch for symmetrical or unsymmetrical C—C bond
what is mass spectrometry?
-x-axis is the mass-to-charge ratio (m/z), z is usually 1 (small molecules typically deal with cations and anions)
-tallest peak is “base peak”
-unfragmented cation of expected molecular weight is “molecular ion” (M+)
-tells relative abundance of ions
mass spectrometer
-two types of ionization
*1. electron impact EI: “hard”, produces fragments
*2. chemical ionization CI: “soft”, usually see molecular ion
isotope patterns
-good clue for finding Br or Cl in a molecule:
*1. 79Br and 81Br are about the same natural abundance, look for equal sized peaks 2 m/z units apart
* 1. 35Cl and 37Cl has a 3:1 peak ratio 2 m/2 units apart
info obtained
-overall, mass spec gives us info about
*molecular weight (using average molecule weight across all isotopes of all atoms in your molecule), “periodic table” mass)
*exact mass (mass of monoisotopic compound, for example, only uses 12C weight instead of averaged weight for all carbon isotopes)= molecular ion (remember, MS can separate all possible isotopes)
*3. molecular formula: the molecular weight can gives us a clue
-highest m/z value peak is usually attributed to the molecular ion
-if every molecule of the molecular ion fragments BEFORE reaching the detector, M+ is not observed
-highest m/z is the assumed mass
what’s a diene?
- diene is two alkenes
- conjugated diene is a molecule with two adjacent double bonds in one molecule (sp^2 and sp^2)
S-cis vs. S-trans
- having an sp^3 center in a diene (non-conjugated) allows for faster rotation
- a diene with adjacent carbons that are sp^2 and sp^2 (conjugated) can’t freely rotate
- since conjugated dienes have delocalized electrons, each pi bond is significantly less stable than pi bonds in unconjugated diene
- heats of hydrogenation for conjugated diene is less than the heat of hydrogenation for the non-conjugated alkene (bc non-conjugated is less reactive) (conjugated is more reactive)
- s refers to sigma bond between the adjacent sp^2 centers
- can convert s-trans to s-cis and vice versa
energy difference is crucial!
- s-trans is MORE stable than s-cis because alkene are far apart from each other while s-cis has the alkenes right next to each other
- mostly work with s-cis
-s-cis is important for more downfield reactivity
“seeing” a difference in extended polyenes
- longer alkene chain has longer maximum wavelength
- as energy decreases, the wavelength goes up so the longer chain absorbs into a higher wavelength
- as you have more alkenes in a row, the energy gap between LUMO and HOMO becomes higher and alkene becomes more reactive
-energy is proportional to the inverse of the wavelength
diene+dieneophile
-we are making a six membered ring
-reaction involves a diene (electron rich) and dienophile (electron poor)
-it’s a concerted mechanism (happens all at once, arrows move together)
-4+2-> gives a Diels-Alder
-to get the most electron poor dienophile, we want electron withdrawing groups
Diels-Alder “bicycle”
-a diels-alder occurs when there is withdrawing groups on the dienophile
-with no withdrawing groups, the ring is locked into place and can’t react with diels alder
-acyclic
back to conformation
-there is no s-trans option because s-trans is NOT reactive (even though s-trans is more stable, it is not reactive)
-must have an s-cis so it’s locked in place
stereochemistry
-stereochemistry stays the same from the alkene dienophile to the product
*if originally cis, groups must be syn
*if originally trans, groups must be anti
more complex
-it is preferred when the R’s are tucked under (endo)
-it is less preferred when the R’s are pointing away (exo)
-try the cube trick to figure out the way the R’s are pointed
alkene+H–X
- addition of HX (1 equivalent) to a diene looks a lot like addition of HX to a regular alkene w/ a slight twist (resonance can occur)
- what is the ratio of our products (trying to find the answer to this)
product ratio depends on carbocations stability
- we know primary<secondary<tertiary (tertiary is the most stable bc it is the more substituted carbocation) (more R groups=more stable) -diene make things more interesting
- secondary allylic carbocation is about as stable as a tertiary cation bc there is resonance in the allylic carbocation
- tertiary allylic carbocation is the most stable of them all (even more stable than a tertiary cation)
diene+H–X product
- at cold temps, we favor external alkene with MORE substituted halide
- at higher temps, we favor internal alkene with LESS substituted halide
product classification
- kinetic product: formed FASTER, favored at lower temps (products with terminal alkenes)
- thermodynamic product: more stable product, favored with heat (products with internal alkenes)
- less stable carbocation has to go through HIGHER transition barrier to get to more stable thermodynamic product
- more stable carbocation has to go through LOWER transition barrier to get to less stable kinetic product
- kinetic forms irreversibly
- thermodynamic equilibrates to some ratio of the 2 products and is reversible
- thermodynamic control: favors thermodynamic product
- kinetic control: favors kinetic product
visualizing kinetic vs. thermodynamic product
-kinetic control:
*1. kinetic product is favored (can still form thermo product but small)
*2. forms irreversibly
-thermodynamic control:
*1. reversible (equilibrate)
*2. thermodynamic product is favored (can still form kinetic product but only small)
- the more stable carbocation forms really fast but it leads to less stable kinetic product (just in this example)
- less stable carbocation forms slower but it leads to the more stable thermodynamic product (just in this example)
when do we get major of each type of product?
-kinetic product: lower temp leads to whatever intermediate forms fastests (non-reversible), doesn’t always give most stable product
-thermodynamic product: higher temp allows for reversibility; via high barrier, ultimately gives most stable product
