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alkane physical properties
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increased MW of alkanes...
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EK Orgo 2
Hydrocarbons, Alcohols, and Substitutions
| Question | Answer |
|---|---|
| alkane physical properties | bp increases with added carbons; branching lowers bp; mp of unbranched increase with increased MW; lowest density of all organics; density increases with MW; almost totally insoluble |
| increased MW of alkanes... | generally increases bp and mp |
| increased branching of alkanes... | generally decreases bp and increases mp |
| ring strain | can occur on some carbon-carbon bonds in a cycloalkane if they are bended away from the normal 109.5 of the sp3 carbon and cause crowding; zero for cyclohexane and increases if ring becomes larger or smaller; less ring strain=lower energy=more stability |
| cyclohexane's 3 conformers | 1. chair - predominates at RT (lowest energy bc two H's on each C are oriented in diff directions) 2. twist 3. boat |
| equitorial H's | H's projecting outward from the center of the ring; position where substituent position is preferred |
| axial H's | H's projecting up or down; this position is often where crowding occurs with substituents and will create instability/high energy |
| combustion | alkane reaction with oxygen - violent reactions that need sufficiently large energy of activations; occurs at high temperature, but once generated can be self perpetuating; radical reaction |
| combustion rxn | CH4 + 2O2 --> (flame) CO2 + 2H2O + heat |
| heat of combustion | change in enthalpy of a combustion rxn; if higher, the energy of molecule is higher, and less stable the molecule |
| halogenation | addition of halogen to an alkane; its mechanism -- free radical chain reaction; exothermic process |
| free radical | active, reacting species formed when light or heat hits diatomic halogen homolytically cleave so that each atom retains 1 e' from the broken bond |
| free radical chain rxn | 1. initiation - X2 is cleaved to 2 X.'s 2. propagation - X. removes H from alkane -> alkyl radical which can react with X2 to create free radicals & continue the rxn 3. termniation - alkyl radial combines with each other or X.'s combine with each other |
| stability of alkyl radical | 3 > 2 > 1 > methyl |
| reactivity of halogens (most to least) | F > Cl > Br > I; where most commonly Cl or Br bc F is too explosive and I just won't react |
| selectivity of halogens (most to least) | (how selective a halogen radical is when choosing a position on an alkane - will choose the most stable alkyl radical) I > Br > Cl > F |
| multi-halogenated products | results with increased [halogen] where dilute solns result in monohalogenated products bc halogen radical more likely to collide with alkane rather than alkyl halide |
| alkene | pi bonds make alkenes more reactive and more acidic than alkanes; more substituted alkenes lead to higher thermodynamic stability; attractive to electrophiles (pi bond) |
| alkene physical properties | - increased MW --> increased BP - increased branching --> increased BP - slightly soluble in h2o - lower density than h2o |
| alkene synthesis | by elimination rxn (1 or 2 functional groups eliminated/removed to form dbl bond) eg: ROH dehydration, dehydrohalogenation OR catalytic syn addition of alkyne |
| alcohol dehydration | E1 rxn, alcohol forms alkene in the presence of concentrated acid |
| E1 | elminiation rxn where the rate depends on one species (eg: alcohol deyhydration - rate depends on [ROH]) |
| steps of R-OH dehydration | 1. acid protonates -OH group on ROH to create good LG 2. H2O LG drops off resulting in a carbocation 3. rearrangment to the most stable carbocation, if necessary 4. H2O deprotonates carbocation to result in alkene |
| carbocation stability (most to least) | 3 > 2 > 1 > methyl |
| Saytseff rule | major product of elimination will be the most substituted alkene |
| dehydrohalogenation | can be E1 (no strong base) or E2 (high concentration of strong, bulky base) |
| dehydrohalogenation via E1 | 1. halogen drops off alkane to result in carbocation 2. weak base abstracts alpha H to form alkene |
| dehydrohalogenation via E2 | (one step) strong base removes H+ resulting in alkene as the halogen drops off |
| role of strong bulky base in E2 reaction | prevents SN2 rxn but if too bulky Saytzeff rule is violated and the least substitued alkene results |
| catalytic hydrogenation | syn addition to alkene using heterogenous catalyst (H2 on Ni, Pd, or Pt) so that the H and alkene adsorb to sruface of the catalyst; exothermic with high energy of activation |
| heats of hydrogenation | can be used to measure relative stabilities of alkenes --> if lower, then alkene is more stable |
| oxidation of alkenes | can produce glycols or cleave the alkene at the double bond |
| oxidation of alkynes | can produce carboxylic acids under ozonolysis |
| electrophillic addition | 1. electrophile attacks double bond to create carbocation 2. carbocation attacks the leftover ion (eg: Br-) to result in addition of that ion to the carbocation |
| electrophile | e' loving species, least + charge |
| Markovnikov's rule | (for hydrogen halide addition) H will add to least substituted carbon of dbl bond |
| anti-Markovnikov addition | happens in the presence of peroxides where the halide, NOT THE H, will add to the least substituted carbon |
| which are the most reactive alkenes in electrophillic addition? | the most thermodynamically stable bc they have the lowest activation energy when forming carbocations |
| concentrated acid and heat help react ___ to ____ | alcohols to alkenes and water |
| dilute acid and cold help react ____ to ___ | alkenes and water to alcohols |
| hydration of an alkene | driven by low temp and dilute acid, follows markovnikov's rule (reversal of dehydration of alcohol) alkene + H2O --> alcohol |
| oxymercuration | follows Markovnikov's rule but rarely results in carbocation rearrangment 1. Hg(OAc)2 --> partial dissociation to +Hg(OAc) 2. +Hg(OAc) + H2O + alkene will add to double bond in anti-addition fashion |
| anti-addition | addition from opposite sides of the double bond |
| demurcuration | takes the product of oxymercuration with a reducing agent (NaBH4) and base to remove Hg to form an alcohol 4(oxymercuration product) + NaBH4 + 4OH- --> 4R-OH + NaB(OH)4 + 4Hg + 4-OAc |
| hydroboration | alkene + BH3 --> alkane-BH2 --> (peroxide, base) alcohol anti-Markovnikov and syn addition to produce R-OH from alkene |
| anti-markovnikov reactions | halogen additions to alkenes in the presence of peroxides, hydroboration |
| markovnikov reactions | halogen addition to alkenes, hydration of alkenes/alkynes, oxymercuration/demurcuration, formation of halohydrin (X adds to least substituted C) |
| halogenenation of alkene | formation of vic-dihalide (2 halogens connected to adjacent carbons)via anti-addition |
| benzene | undergoes substitution NOT addition, flat molecule, stabilized by resonance |
| ortho positions on benzene ring | 1,2 or 1,6 directed by e' donating groups |
| meta positions on benzene ring | 1,3 or 1,5 directed by e' withdrawing groups |
| para positions | 1,4 (opposite sides of ring) directed by e' donating groups |
| strongly e' donating | -O-, -OH, -NR2 (oxygen ion, alcohol, amine with 2 R groups) |
| strongly e' withdrawing | -NO=O, -NR3, -CCl3 (nitro group, amine with 3 R groups and +N, carbon tetrachloride |
| moderately e' donating | -OR (ether) |
| weakly e' donating | alkyl group |
| moderately e' withdrawing | -CR=O, CH=O, COOR, COOH, SOOOH, -C=-N (carbonyls, sulfur oxygen containing, cyanide) |
| weakly e' withdrawing | halogens |
| electron withdrawing group | when occupying 1 position on benzene ring, it will deactivate (makes less reactive) the ring and direct substituents to the meta position |
| electron donating group | when occupying 1 position on the benzene ring, it will activate (makes more reactive) the ring and direct substituents to the ortho and para positions-- stabilizes double bonds bc alkenes withdraw e' through their bonds |
| substition rxns | can be unimolecular (SN1) or bimolecular (SN2) which represents the order of the rate law - SN1 rate of substitution depends on 1 molecule and SN2 rate depends on 2 molecules |
| SN1 | -2 steps -rate = k[S] -carbocation intermediate -prefers tertiary carbons -prefers protic solvents -produces racemic mixture -often accompanied by E1 rxns bc Nu:- can act as a base to abstract H+ from carbocation |
| SN2 | -1 step -2nd order kinetics -involves transition state -prefers primary carbons -prefers aprotic solvents -produces optically active products and inversion of stereochemistry |
| SN1 mechanism | halogen ion drops off substrate creating carbocation -> Nu:- attacks carbocation --> Nu substituted for halogen (both enantiomers) |
| SN2 mechanism | Nu:- attacks carbon on one side as the LG breaks free from substrate |
| why don't SN2 rxns typically occur with tertiary substrates? | tertiary carbons sterically hinder the nucleophile |
| why don't SN2 rxns typically occur in the presence of a strong base? | if also stericaly hindered, an E2 rxn can occur |
| why don't SN2 rxns typically occur with bulky nucleophiles? | steric hinderance |
| nucleophilicity | -degree of wanting to donate e's -decreases L->R across periodic table and bottom to top -more so if negative charge and polarizable -base always stronger Nu:- than its conjugate acid but BASICITY NOT EQUAL TO NUCLEOPHILICITY |
| polar protic solvents | polar solvents that can hydrogen bond, stabilize nucleophile and any carbocation that forms--> increase rate of SN1 and decrease rate of SN2 |
| polar aprotic solvents | (polar solvents that can't H bond) increase SN2 rxns and inhibit SN1 rxns bc they can't form strong bonds with ions |
| leaving groups | -the best are those that are stable when they leave - generally, the weaker the base, the better the LG -e' withdrawing effect & polarizability also make good LGs |
| physical properties of alcohols | -increased MW increase BP&MP -branching decreases BP, unclear effect on MP -BP much higher than alkanes (H bonds) -more soluble in water than alkanes/alkenes bc OH increases polarity & allows H bonding -longer carbon chain, the less soluble the alcoho |
| alcohols as acid (strongest to weakest) | methyl > 1 > 2 > 3 the conjugate bases that can absorb more (-) charge are more stable and its corresponding acid is stronger **H can absorb (-) charge better than methyl groups** |
| Rxns to synthesize alcohols | -hydration of alkane -oxymercuration/demurcuration -hydroboration -nucleophilic substitution -Grignard reagents -reduction synthesis with NaBH4 or LiAlH4 |
| Grignard reagents | R-MgX (highly polarized carbon metal bond where the carbon is more electronegative than the metal so it takes on a strong partial (-) charge making it a STRONG NUCLEOPHILE AND BASE) need to be made in ether- incompatible with water |
| Grignard synthesis of an alcohol | R-MgX + R-CR=O --> CR3O- - +MgX --> CR3OH + XMgOH extends C skeleton, will react in similar fashion with C=N, C=-N, S=O, N=O and can deprotonate O-H, N-H, S-H, -C=-C-H nucelophillic carbon attacks carbonyl and after acid bath, will produce an alcoho |
| Reduction synthesis | using NaBH4 or LiAlH4, the H- will attack the carbonyl to reduce aldehydes and ketones to alcohols only LiAlH4 is strong enough to reduce esters and acetates |
| why is it more difficult to reduce esters and acetates than ketones and aldehydes? | bc the grp attached to the carbonyl of the ester or acetate is a stronger e' donor than an akyl grp or H. by donating e's more strongly, it reduces the (+) charge on the arbonyl making it less attractive to the nucleophile |
| how alcohols act as nucleophiles.. | thru nucelophillic addition or substitution; the lone pairs on oxygen can attack positive charge |
| oxidation in orgo rxns | loss of H2, addition of O or O2, addition of X2 |
| reduction in orgo rxns | addition of H2, loss of O or O2, los of X2 |
| neither oxidation or reduction in orgo rxns | addition or loss of H+, H2O, HX, etc |
| oxidation of alcohols | only primary and secondary ROH's can be oxidized (primary -> aldehydes; secondary ->ketones) |
| oxidizing agents | K2Cr2O7 KMnO4 H2CrO4 O2 Br2 |
| reducing agents | LiAlH4 NaBH4 H2 + pressure |
| alcohols to alkyl halides | R-CH2-OH + HX --> R-CH2-OH2 + X- --> R-CH2-X protonates OH group to make good LG (water) where the alcohol is acting as an electrophile and halogen as the nucleophile; can also be accomplished with P-halides (PBr3, PI3, PCl3 as SN2 mechanism) or SOCl2 |
| formation of sulfonate | via nucleophillic substitution where ROH acts as nucleophile, retention of configuration |
| sulfonate | -sulfur double bonded to two O's, single bond to one O and one R group - common ones are tosylates and mesylates - act as weak bases and excellent leaving groups in SN1 or SN2 rxns |
| pinacol rearrangement | dehydration of alcohol but results in a ketone or aldehyde instead of the typical alkene when reacting vicinal diol (diol on adjacent C's) and hot sulfuric acid |
| pinacol rearrangement mechanism | 1. acid protonates 1 OH group and removed by the acid to form carbocation 2. methyl group can move to form more stable carbocation 3. carbocation will have resonance that forms dbl bonded oxyen (pinacalone) 4. H removed, pinacolone + acid catalyst resu |
| ethers | relatively non reactive, polar, can only H bond with other componds that have H attached to N, O, or F (not themselves), roughly soluble in water, dissolves organic cmpds easier, has low BP |
| rxns with ethers | -cleaved by haloacids (either HI of Hbr) to form corresponding alcohol and alkyl halide -oxidized to peroxides |
| epoxide | -three membered cyclic ethers, more reactive than typical ethers due to strain created by small ring -react with water in the presence of acid catalyst to form diols in anti-addition |
| acid strength of functional groups (weak to strong) | alkane < alkene < H2 < NH3 < alkyne < aldehyde < alcohol < water < carboxylic acid |
| hydration of alkyne | results in -OH group added to most substituted carbon on alkene (markonikov) |
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miniangel918
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