Lecture5: 123.101

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Information about Lecture5: 123.101

Published on March 3, 2009

Author: gjr2323

Source: slideshare.net

Unit One Part 5:intermolecular forces we have looked at the bonds in molecules, now turn our attention to the bonds / forces between molecules

5Unit OnePart of molecules (pg56-58)PropertiesIntermolecular forces (pg59-66)Solubility (pg66-68)

Gecko feetAutumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256

Gecko feet how do geckos walk on walls? to understand this cool phenomena we have to understand the attraction between molecules...Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256

O H O2-methylpropan-2-ol ethoxyethane tert-butanol diethyl ether C4H10O C4H10O mp 26°C mp by looking at –116°C lets start these two simple molecules...they are structural isomers...

O H O2-methylpropan-2-ol ethoxyethane tert-butanol diethyl ether C4H10O C4H10O mp 26°C mp –116°C same atoms...but very different properties...why?

why are the physicalcharacteristics so different?

is it the bondsin the molecule? yes... C–O–C versus C–O–H ...but...

...or is itsomething more?

pentane 2-methylbutane 2,2-dimethyl C5H12 C5H12 propanebp 36.2˚C bp 28˚C C5H12 three more bp 9.6˚C isomers...this time no change in functional groups...

pentane 2-methylbutane 2,2-dimethyl C5H12 C5H12 propanebp 36.2˚C bp 28˚C C5H12 bp 9.6˚C ...but they still have different physical properties!

similar bonds, but very different properties

need to understandforces between molecules

...of course, this is controlled bythe bonds in the molecules...

...and the electrons (of course)...of course, this is controlled bythe bonds in the molecules...

before we can look at the forces we need to define a few terms......of course, this is controlled bythe bonds in the molecules...

Bond dipoles(separation of charge) H Cl δ+ δ– we now know that electrons are not shared evenly between atoms... δ+ δ– = H Cl H Cl

Bond dipoles(separation of charge) H Cl δ+ δ– δ+ δ– = ...they are attracted towards the most H Cl H Cl electronegative atom and the bond is said to be polarised

Bond dipoles(separation of charge) H Cl δ+ δ– the bond dipole refers to the difference in charge on each atom and their separation δ+ δ– = H Cl H Cl

H 2.1 Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Na Mg Al Si P S Cl Pauli scale of 0.9 1.2 1.5 1.8 2.1 2.5 3.0 electronegativities K Ca Br allows us to predict0.8 1.0 2.8 bond polarity... Rb Sr I 0.8 1.0 2.5 EN Bond Type difference Examples Calculation ionic > 1.7 NaCl 3.0(Cl) - 0.9(Na) = 2.1 CH3O–H 3.5(O) - 2.1(H) = 1.4polar covalent 0.5 – 1.7 H–Cl 3.0(Cl) - 2.1(H) = 0.9 covalent 0 – 0.4 CH3–H 2.5(C) - 2.1(H) = 0.4 Pg H–H 2.1(H) - 2.1(H) = 0.0 35

electro- negative 1 18 H 2 13 14 15 16 17 He 9 Li Be B C N O F Ne F Na Mg 3 4 5 6 7 8 9 10 11 12 Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 87 We do not have to remember Fr Ra Ac Fr all the values, just the general trend (and eventually the effect of different functional groups)electro-positive

Molecular dipoles(su m of b on d dipo les) if we add all the individual bond dipoles together we Cl get the... H C H H polar molecules

Molecular dipoles(su m of b on d dipo les) ...the molecular dipole or dipole Cl moment H C H H dipole moment polar molecules

Molecular dipoles(su m of b on d dipo les) Cl H C H O H H dipole H moment dipole ...compounds with moment a dipole moment are polar said to be polar molecules molecules

Molecular dipoles(su m of b on d dipo les) if the bond dipoles cancel each other out (thats why shape is important), the Cl molecule will have no dipole moment and is Cl C Cl non-polar Cl no dipole moment non-polar m o l e c u l e s

Inductive effects ( lo n g ran ge e ffect s) a functional group that attracts electrons is an electron withdrawing group (EWG) H2 H2 H2 C >C> or or δ+ C δ– H3C Cl H 3C Cl H3C δ+ Clelectron-withdrawing group

Inductive effects ( lo n g ran ge e ffect s) first it causes a bond dipole in its own bonds H2 H2 H2 C >C> or or δ+ C δ– H3C Cl H 3C Cl H3C δ+ Clelectron-withdrawing group

Inductive effects ( lo n g ran ge e ffect s) ...and this dipole induces a bond dipole in bonds next to it H2 H2 H2 C >C> or or δ+ C δ– H3C Cl H 3C Cl H3C δ+ Clelectron-withdrawing group

Inductive effects ( lo n g ran ge e ffect s) the further from the functional group the smaller this polarisation H2 H2 H2 C >C> or or δ+ C δ– H3C Cl H 3C Cl H3C δ+ Clelectron-withdrawing group

Inductive effects( lo n g ran ge e ffect s) a group that pushes electrons away (alkyl) is an electron donating group H 3C > Celectron-donating group

...and the inductiveeffect in action...

Acidity O H O H H H O OH3C O H H3C O H we measure acidity by how readily a high low compound looses H+ pKa pKa

Acidity the more stable the anion the more readily the compound looses O H H+ O H H H O OH3C O H H3C O H high low pKa pKa

Acidity O H O H H H O OH3C O H H3C O H the more stable the anion, the more acidic the compound and the high low reaction shifts to the right... pKa pKa

Acidity O H O H H H O OH3C O H H3C O H this is measured by pKa...the lower the pKa the more acidic the high low compound...more about this later in semester pKa pKa

Acidity O H O H H H O OH3C O H H3C O H electron withdrawing groups help stabilise high low negative charges pKa pKa

unit 3 This topic is covered in detail in unit 3 but at the moment all we have to remember is...

high pKa O molecule is basic, this means it wants the H proton H+ or the anion is unstableR O wants H+

low pKa O HR Omolecule with a low pKa will loose a proton H+readily to go from HA to losses H+ H+ and A– or...

low pKa O ...the molecule can stabilise an anion (negative charge) H R Onegative charge stable

Acidity O H O H H H O OH3C O H H3C O H O O O O Cl H Cl H H Cl H O « OH3C O < O Cl Cl Cl pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70 CO2H Cl Cl CO2H CO2H Cl pKa = 2.85 pKa = 4.05 pKa = 4.50

Acidity O H wants H O+ H H H O OH3C O H H3C O H O O O O Cl H Cl H H Cl H O « OH3C O < O Cl Cl Cl pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70 CO2H Cl Cl CO2H CO2H Cl pKa = 2.85 pKa = 4.05 pKa = 4.50

Acidity the more electron O H withdrawing groups the O H H more stable the negative O O H charge and so the pKa isH3C O H lower and the compound H3C O H is more acidic O O O O Cl H Cl H H Cl H O « OH3C O < O Cl Cl Cl pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70 CO2H Cl Cl CO2H CO2H Cl pKa = 2.85 pKa = 4.05 pKa = 4.50

Acidity losses H H+ H O O H H O OH3C O H H3C O H O O O O Cl H Cl H H Cl H O « OH3C O < O Cl Cl Cl pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70 CO2H Cl Cl CO2H CO2H Cl pKa = 2.85 pKa = 4.05 pKa = 4.50

Acidity O R O O H stable O H H H O OH3C O H H3C O H O O O O Cl H Cl H H Cl H O « OH3C O < O Cl Cl Cl pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70 CO2H Cl Cl CO2H CO2H Cl pKa = 2.85 pKa = 4.05 pKa = 4.50

Acidity O H O H H H O OH3C O H H3C O H the further away the electron withdrawing group from the O O O Onegative charge the smaller the effect Cl H Cl H H Cl H O « OH3C O < O Cl Cl Cl pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70 CO2H Cl Cl CO2H CO2H Cl pKa = 2.85 pKa = 4.05 pKa = 4.50

you do not need !to learn these values!

how does all this effect intermolecular forces??so what intermolecular forces are there?

covalent bond (strong)govern reactions H Cl H Cl forces between molecules are relatively weak BUT very important! intermolecular attraction (weak) physical properties

sorry... the next slide is awful (in all the worst senses of the word)

typical energy interaction (kJmol–1) ionic-ionic (ionic bond) 250intramolecular carbon-containing covalent bond 350 forces oxygen-hydrogen covalent bond 460 hydrogen (H-) bond 20intermolecular ion-dipole 15 forces dipole-dipole 2 London (dispersion) 2 shows just how weak intermolecular forces are by comparison...

transferable skill...

• Tables are rarely of any use in a presentation...• Lots of text on a PowerPoint (or Keynote) slide is not only really dull but looks crap and is second only to the use of...• ...bullet points in making you (and me) a little sleepy• So stick to pictures (and a lot of preparation)

don’t get me started on the differences between... only the first is acceptable to me (butmost people can’t see these thesethe difference between 1 & 3) learn to do presentations right...it’ll serve these you well!

why does NaCldissolve in water? ...or, now lets look at the intermolecular forces (at last)!

H δ+ Cl Hδ+ δ-- O Na O δ-- H δ+ Hδ+ δ– δ+ δ+ δ– δ+ Cl– δ+ δ+ δ+ δ–Ion-dipoleforces (15 kJmol-1)

H δ+ Cl Hδ+ δ-- O Na O δ-- H δ+ Hδ+ δ– δ+ δ+ interaction of an ion (chloride) δ– δ+ Cl– δ+ δ+ δ+ δ–Ion-dipoleforces (15 kJmol-1)

H δ+ Cl Hδ+ δ-- O Na O δ-- H δ+ Hδ+ δ– δ+ δ+ δ– δ+ Cl– δ+ with a δ+ compound with a permanent δ+ δ– dipole (water)Ion-dipoleforces (15 kJmol-1)

H δ+ Cl Hδ+ δ-- O Na O δ-- H δ+ Hδ+ δ– δ+ δ+ δ– δ+ Cl– δ+ δ+ δ+ δ– relativelyIon-dipole strong...hence salt dissolvesforces (15 kJmol-1)

H δ+ Cl Hδ+ δ-- O Na O δ-- H δ+ Hδ+ δ– δ+note that water δ+ interacts with both anion and cation...really δ– δ+ helps Cl– δ+ δ+ δ+ δ–Ion-dipoleforces (15 kJmol-1)

δ– HO Hδ+ O Na O δ– H δ+ organic compounds can do the same (but only O–H bond)Ion-dipoleforces (15 kJmol-1)

δ+ δ– δ+ δ– two molecules with a dipole will interact... δ+ δ– weakly δ– δ+Dipole-dipoleforces (≈ 2 kJmol-1)

Polarmix polarmolecules / H3C Cl δ+ C O δ– δ+ H C Cl δ– H3C Cl this is why two polar molecules (like acetone & chloroform) will δ+ δ– δ+ δ– δ+ δ– mix...they are attracted δ– δ+ –δ +δ δ– δ+ to each other δ+ δ– δ+ δ– δ+ δ– δ+ δ– δ+ δ– δ+ δ– δ+ δ– δ– δ+

Polardo/not mixmolecules non-polar H H2 H2 δ+ O δ– C C CH3 H3C C C H H2 H2 but polar molecules won’t mix with non– polar...polar molecules run an exclusive club δ+ δ– δ+ δ– δ+δ– and they won’t let δ– δ+ δ– δ+ anyone else in... δ– δ+ δ– δ+ δ– δ+ δ+ δ–

δ– H2 H3C δ+ O CH3 H3C C H2 C CH3 H3C C H2 CH3 O δ+ CH3 C CH3 δ– H2 propanone butane acetone Mol Wt. 58; bp 56°C Mol Wt. 58; bp –0.6°C permanent dipole no dipole compare these two molecules...similarDipole-dipole size and identical weight...& boiling points

δ– H2 H3C δ+ O CH3 H3C C H2 C CH3 H3C C H2 CH3 O δ+ CH3 C CH3 δ– H2 propanone butane acetone Mol Wt. 58; bp 56°C Mol Wt. 58; bp –0.6°C permanent dipole no dipoleDipole-dipole& boiling points ...but very different boiling points...

δ– H2 H3C δ+ O CH3 H3C C H2 C CH3 H3C C H2 CH3 O δ+ CH3 C CH3 δ– H2 propanone butane acetone Mol Wt. 58; bp 56°C Mol Wt. 58; bp –0.6°C permanent dipole no dipole this is because acetone has dipole– dipole attractions holding moleculesDipole-dipole together and butane doesn’t& boiling points

...one intermolecularforce to rule them all...

δ+ δ– δ+ δ– δ+ δ+Hydrogen H O H Obonding H H hydrogen bond

δ+ δ– δ+ δ– δ+ δ+Hydrogen a special kind of H dipole–dipole interaction...occurs O between... H Obonding H H hydrogen bond

H-bond H X polar bonddono r (X = O, N etc) XH-bond lone pair onacceptor electronegative atom

H-bonding it is responsible for water being so wonderfully odd water’s abnormal propertiesF.W. Starr/Wesleyan Univ.

δ– H δ+ O δ+H H δ+ δ– O H O δ– δ+ H δ+ Hδ+ δ– δ+ O δ– O H δ+ H H δ+ H δ+ three molecules of similar sizeH2O (MW=18): boiling point 100°C and / or shapeH2S .(MW=34): boiling point –60°CCH4 (MW=16): boiling point –162°C

...yet water has phenomenally high δ– H δ+boiling point...all due O to H–bonding! (also explains ice) δ+H H δ+ δ– O H O δ– δ+ H δ+ Hδ+ δ– δ+ O δ– O H δ+ H H δ+ H δ+H2O (MW=18): boiling point 100°CH2S .(MW=34): boiling point –60°CCH4 (MW=16): boiling point –162°C

Methanol methanol only has one O–H bond so can only form one H-bond so has much lower bp (less attraction) CH3 δ+ Oδ–δ+ δ– H H O δ– CH3 O δ+ H3C H H δ+ O δ– CH31 hydrogen bondboiling point 62°C

H-bondingcarboxylic acids carboxylic acids are also capable of forming H– bonds between OH (H-bond donor) and C=O (polarised so lone pair is H-bond acceptor) δ– δ+ δ– O H O δ+ H3C CH3 O H O δ– δ+ δ–

H-bondingcarboxylic acids- solubility ...H-bonding allows some acids to dissolve in water as good attraction (hence we can have vinegar (shown) δ+ H O δ– and glacial acetic acid (very different) δ–O H H3C H δ+ δ– δ+ δ– H O H O δ– H O H δ+

Hydrogen bonding vital in biology...

proteinsecondarystructure

H O R2 H H N N N O H R1 Hδ+ O δ– δ– O δ+ H H R4 N N R3 H O HHydrogen β-sheets and α-helix etc are all formed by H-bonding between amidesbonding

without H-bonding no DNA double helix!©Benny Herudek 3D Hifi - High Fidelity 3d Graphics Solutions DNA

H N N H O CH3 N N H N N N O adenine thymineHydrogen Watson-Crick base pairing (or whatever the biochemists call it)bonding

all molecules can interact... don’t need a dipole to interact because...

London (van der Waals or dispersion) forces here we have two molecules...

London (van der Waals or dispersion) forces δ– δ+ ...chance allows the electrons of one molecule to bunch at one end causing an imbalance of electrons... momentary dipole

London (van der Waals or dispersion) forces δ– δ+ momentary ...a disturbance in the dipole force...or setting up a momentary dipole...

London (van der Waals or dispersion) forcesδ– δ+ δ– δ+ ...as electrons don’t like each other this new bunch repulse electrons in a near by molecule induced momentary and set up an induced dipole... dipole dipole

London (van der Waals or dispersion) forces this causes dipole–dipole attraction (momentarily)...but it will soon stop as the electrons are always on the moveδ– δ+ δ– δ+ attraction induced momentary dipole dipole

Largersurface area the larger the molecule, the bigger the surface area and the more electrons involved... Bigger the force

Largersurface area ...this means a bigger momentary dipole can be formed... Bigger the force

Largersurface area and thus greater attraction Bigger the force

Largersurface area H H H H H H H H C C C H C H C C C H H H H H H H H methane hexane CH4 (MW=16) C6H14 (MW=86) mp –182°C; bp –164°C mp –95°C; bp 69°C gas at rt liquid at rt H H H H H H H H H H H H H H H H H H H H H C C C C C C C C C C C C C C C C C C C C H H H H H H H H H H H H H H H H H H H H H eicosane C20H42 (MW=282) mp 36°C; bp 343°C Bigger ...this goes solid at rt someway to explaining why bigger molecules have higher boiling point (bp)...but other factors also involved the force

Largersurface area pentane 2,2-dimethylpropane bp 36°C bp 9.5°C and at last explains the difference between isomers Bigger the force

Gecko feet ...and it is van der Waals forces that are responsible for Gecko’s ‘sticky’ feet!Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256

solvationinteraction of molecules & solvent

propanol H H O Opropanol & water mix H as they can interact by H-bonds... H H O watersoluble

alcohol (OH) makes propanol means water loving in Latin (I think)hydrophilic

hexane H H O H H no interaction Obetween molecules Oso hexane floats on waterH H O H H waterinsoluble

hexane H H O H H O O H H polar molecules O (water) do not mix H H with non-polar water molecules (hexane)insoluble

non-polar grease makes hexane means water fearing (hating)hydrophobic

polardissolves ...conversely, non-polar compounds dissolve polar other non-polar compounds

hydrophilicdissolves doesn’t really need explanation... hydrophilic

hydrophobicdissolves hydrophobic

propanoic acid✔ H H O O H H O O H H H O H-bonding allows molecules to interact, thus mixing...hydrophobic ethyl water chain too small to effect interaction

hydrophilic✔H H O O H H O O H H H O water

butanoic acid✔ H H O O H H O O H H H O addition of one more carbon to chain does not make much difference...butanoic water acid still soluble in water

hexanoic acid /✔✘ O H H O O H H H O but a pentyl chain is pushing our luck...non- polar chain starts to effect water solubility and only a little will dissolve

✘decanoic acid O H H O O H H H O get to a point where the blob of grease controls the properties and overcomes the H- water bond interactions...

✘hydrophobic O H H O O H H H O ...to give us a compound that will not mix with water as too much of it is non- water polar. So decanoic acid is hydrophobic

✘ sugar O OH HO HO OH OH we can have thereverse...a very polar molecule will notdissolve / mix with a non-polar solvent hexane

✘ sugar O OHHO HO OH OH ...each OH group is polar and thus will not mix with hexane hexane...

✘ sugar - hydrophilic O OH HO HO OH OH very little interactions between two molecules so they do not mixhexane - hydrophobic

✔ sugar O OHHO HO OH OH ...obvious I hope (sugar dissolves in coffee!)...due to all the OH groups H-bonding water to water

✔ sugar H H O O H H O H H O O HHO H O H H H O O lots of hydrogen bonding H O H H H O O H O H H water

doesn’t this look good!especially after all this chemistry

what have we learnt? all about intermolecular forces

readpart 6 ©paulbence@flickr

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