郑州大学有机化学.ppt

第九章第九章 测定有机化合物结构的物理方法测定有机化合物结构的物理方法波谱学波谱学(Spectroscopy)基础基础Organic structures can be determined accurately and quickly by spectroscopyThe electromagnetic spectrumX-rays(X-射线射线)give bond lengths and anglesNuclear magnetic resonance(核磁共振核磁共振)tells us about the carbon skeleton of the moleculeInfrared spectroscopy(红外光谱红外光谱)tells us about the types of bond in a moleculeMass spectrometry（质谱）（质谱）determine mass of molecule and atomic compositionuX-ray crystallography has its limitationsX-ray crystallography works by the scattering of X-rays from electrons and requires crystalline solids.If an organic compound is a liquid or is a solid but does not form good crystals,its structure cannot be determined in this way.uX-ray is the final appealdraw saturated carbon chains as zig-zagsand not in straight lines with 90or 180bond anglesHO2C(CH2)4 CO2Hhexanedioic acidshape of hexanedioic acidX-ray crystal structure of hexanedioic acidInfrared spectroscopyInfrared(IR)spectroscopy detects the stretching and bending of bonds rather than any property of the atoms themselves.It is particularly good at detecting the stretching of unsymmetrical bonds of the kind found in functional groups such as OH,C=O,NH2,and NO2.When the carbon skeleton of a molecule vibrates,all the bonds stretch and relax in combination and these absorptions are unhelpful.However some bonds stretch essentially independently of the rest of the molecule.This occurs if the bond is either:much stronger or weaker than others nearby,or between atoms that are much heavier or lighter than their neighboursf-force constant(more or less the bond strength)-frequence of vibration-reduced massThere are four important regions of the infrared spectrumThe XH region distinguishes CH,NH,and OH bondsThe reduced masses of the CH,NH,and OH combinations are all about the same.Any difference between the positions of the IR bands of these bonds must then be due to bond strength.4000 3000 2000 1500 1000IR bands for bonds to hydrogenBond Reduced mass IR frequency,cm-1 Bond strength,KJ mol-1C-H 12/13=0.92 2900-3200 CH4:440N-H 14/15=0.93 3300-3400 NH3:450O-H 16/17=0.94 3500-3600a H2O:500a When not hydrogen bonded.Bonds to hydrogenO-HN-HC-HTriple bondsCCCNDouble bondsC=CC=OSingle bondsC-OC-FC-ClThe OH bands occur at higher frequency,sometimes as a sharp absorption at about 3600 cm1.More often,you will see a broad absorption at anywhere from 3500 to 2900 cm1.This is because OH groups form strong hydrogen bonds that vary in length and strength.The sharp absorption at 3600 cm1 is the non-hydrogen-bonded OH and the lower the absorption the stronger the H bond.NH,NH2O-HThe double bond region is the most important in IR spectraIn the double bond region,there are three important absorptions,those of the carbonyl(C=O),alkene(C=C),and nitro(NO2)groups.All give rise to sharp bands:C=O to one strong(i.e.intense)band anywhere between 1900 and 1500 cm1;C=C to one weak band at about 1640 cm1;and NO2 to two strong(intense)bands in the mid-1500s and mid-1300s cm1.SummaryThe strength of an IR absorption depends on dipole momentThe strength of an IR absorption varies with the change of dipole moment when the bond is stretched.If the bond is perfectly symmetrical,there is no change in dipole moment and there is no IR absorption.Obviously,the C=C bond is less polar than either C=O or N=O and is weaker in the IR.Position of band depends on:reduced mass of atoms light atoms gives high frequency bond strength strong bonds gives high frequency Strength of band depends on:change in dipole moment large dipole moment gives strong absorptionWidth of band depends on:hydrogen bonding strong H bond gives wide peakAbsorptions in IR spectra1.指出下列哪对化合物可用红外光谱区别指出下列哪对化合物可用红外光谱区别和和和和和和和和abcd核磁共振核磁共振 NMR1951年年 Arnold 发现乙醇的发现乙醇的NMR信号及其与信号及其与结构的关系结构的关系1945年年 Purcell和和 Bloch发现核磁共振现象，发现核磁共振现象，他们获得他们获得1952年年Nobel物理奖物理奖1953年年 Varian公司试制了第一台公司试制了第一台NMR仪器仪器Richard R.ErnstThe Nobel Prize in Chemistry 1991 for his contributions to the development of the methodology of high resolution nuclear magnetic resonance(NMR)spectroscopy凡是自旋量子数凡是自旋量子数I 0的原子核，都可发生核磁共振的原子核，都可发生核磁共振质量数质量数和和原子序数同时为偶数的原子核原子序数同时为偶数的原子核，其自旋量子数为零，没有磁矩，其自旋量子数为零，没有磁矩，所以所以不发生核磁共振不发生核磁共振Nuclear magnetic resonanceNuclear magnetic resonance(NMR)allows us to detect atomic nuclei and say what sort of environment they are in,within their molecule.An NMR machineMagnetic Resonance ImagingNMR uses a strong magnetic field,and radio wavesI)some atomic nuclei act like tiny compass needles and have different energy levels when placed in a magnetic field.II)The energy levels of atomic nucleus are quantized,and there are only certain specific energy levels it can adopt.This is like allowing our compass needle to point,say,only north or south.III)Some nuclei(e.g.,carbon-12)do not interact with a magnetic field at all and cannot be observed in an NMR machine.The nuclei we shall be looking at,1H and 13C,do interact and have just two different energy levels.When we apply a magnetic field to these nuclei,they can either align themselves with it,which would be the lowest energy state,or they can align themselves against the field,which is higher in energy.IV)Radio-waves are used to flip the nucleus from the lower energy state to the higher state.When the nuclei fall back down to the lower energy level,energy was given out,which was detected.How does it work?intensityfrequency(in parts per million,p.p.m.)100 MHz spectrumSolvent peakTypes of NMR spectrometers:1.Sweep(CW)NMR spectrometers2.Fourier Transform(FT)NMR spectrometersScales of NMRMagnetic field:telsa.The earths magnetic field has a field strength of 2 x 10-5 telas.A typical magnet used in an NMR machine has a field strength of between 2 and 10 telsa.Radio-wave frequency:MHz.Rather than referring to the strength of the magnet in tesla,chemists usually just refer to its operating frequency.A 9.4 T NMR machine is referred to as a 400 MHz spectrometer since that is the frequency in this strength field at which the protons in the reference sample resonate;other nuclei,for example 13C,would resonate at a different frequency,but the strength is arbitrarily quoted in terms of the proton operating frequency.Why do chemically distinct nuclei absorb energy at different frequencies?The variation in frequency for different carbon atoms must mean that the energy jump from nucleus-aligned-with to nucleus-aligned-against the applied magnetic field must be different for each type of carbon atom.The reason there are different types of carbon atom is that their nuclei experience a magnetic field that is not quite the same as the magnetic field that we apply.Each nucleus is surrounded by electrons,and in a magnetic field these will set up a tiny electric current.This current will set up its own magnetic field,which will oppose the magnetic field that we apply.The electrons are said to shield the nucleus from the external magnetic field.If the electron distribution varies from 13C atom to 13C atom,so does the local magnetic field,and so does the resonating frequency of the 13C nuclei.The variation in frequency is known as the chemical shift.Why do the chemical shift scale appear in parts per million(p.p.m.),not in magnetic field units or frequency units?The exact frequency at which the nucleus resonates depends on the external applied magnetic field.This means that,if the sample is run on a machine with a different magnetic field,it will resonate at a different frequency.It would make life very difficult if we couldnt say exactly where our signal was,so we say how far it is from some reference sample,as a fraction of the operating frequency of the machine.The reference sampletetramethylsilane,TMSThe chemical shift,in parts per million(p.p.m.)of a given nucleus in our sample is defined in terms of the resonance frequency as:No matter what the operating frequency(i.e.strength of the magnet)of the NMR machine,the signals in a given sample will always occur at the same chemical shifts.By definition TMS itself resonates at 0 p.p.m.Different ways of describing chemical shiftProton nuclear magnetic resonanceThe differences between carbon and proton NMR1)1H is the major isotope of hydrogen(99.985%natural abundance),while 13C is only a minor isotope(1.1%).2)1H NMR is quantitative:the area under the peak(Integration)tells us the number of hydrogen nuclei,while 13C NMR may give strong or weak peaks from the same number of 13C nuclei.3)Protons interact magnetically(couple)to reveal the connectivity of the structure,while 13C is too rare for coupling between 13C nuclei to be seen.4)1H NMR shifts give a more reliable indication of the local chemistry than that given by 13C spectra.Regions of the proton NMR spectrumProtons on unsaturated carbons next to oxygen:aldehydesProtons on unsaturated carbons:benzene,aromatic hydrocarbonsProtons on unsaturated carbons:alkenesSaturatedCH3CH2CHnext to oxygenSaturatedCH3CH2CHnot next to oxygen10.58.56.54.53.00.0Me4Si 磁各向异性效应磁各向异性效应双键碳上的质子双键碳上的质子烯烃双键碳上的质子烯烃双键碳上的质子位于位于键环流电子产生的感生磁场键环流电子产生的感生磁场与外加磁场方向一致的区域与外加磁场方向一致的区域(去屏蔽区去屏蔽区),去屏蔽效应的结去屏蔽效应的结果果,使烯烃双键碳上的质子的共振信号移向使烯烃双键碳上的质子的共振信号移向稍低的磁场稍低的磁场区区,=4.55.7醛基质子醛基质子=9.410三键碳上的质子三键碳上的质子炔键炔键是直线型是直线型,电子云围绕碳碳电子云围绕碳碳键呈筒型分布键呈筒型分布,形成形成环电流环电流,它所产生的感生磁场与外加磁场方向相反它所产生的感生磁场与外加磁场方向相反,故三故三键上的质子处于键上的质子处于屏蔽区屏蔽区,屏蔽效应较强屏蔽效应较强,使使炔基质子炔基质子的共的共振信号移向振信号移向较高的磁场区较高的磁场区,=23分子中某些基团的电子云排布不呈球形对称，它对邻近氢核产生一个分子中某些基团的电子云排布不呈球形对称，它对邻近氢核产生一个各向异性各向异性的磁场的磁场，从而使某些空间位置上的核，从而使某些空间位置上的核受屏蔽受屏蔽，而另一些空间位置上的核，而另一些空间位置上的核去屏蔽去屏蔽的现象的现象The benzene ring current causes large shifts for aromatic protonsCoupling in the proton NMR spectrumCoupling-nearby hydrogen nuclei interact and give multiple peaks,allows us to look not just at individual atoms but also at the way the C-H skeleton is joined together.The pyrimidine spectrum has two single lines(singlets)because each proton,HA or HX,can be aligned either with or against the applied magnetic field.The cytosine spectrum is different because each proton,say,HA,is near enough to experience the small magnetic field of the other proton HX as well as the field of the magnet itself.原子核之间的自旋相互作用原子核之间的自旋相互作用(即干扰即干扰),叫做叫做自旋偶合。自旋偶合。由自旋偶合引起的谱线由自旋偶合引起的谱线增多的现象增多的现象，叫做，叫做自旋裂分自旋裂分。自旋偶合常只需考虑自旋偶合常只需考虑邻碳偶合邻碳偶合以及以及同碳偶合同碳偶合化学等性质子之间不发生偶合化学等性质子之间不发生偶合两个化学不等性质子相隔三个两个化学不等性质子相隔三个键以上时，不发生偶合键以上时，不发生偶合同碳上的化学不等性质子可偶合裂分同碳上的化学不等性质子可偶合裂分化学等价化学等价:分子中两个分子中两个质子处于相同的化学环境质子处于相同的化学环境coupling constant J-hertzIf one proton HA interacts with two protons HX,it can experience three states of proton HX.Both protons HX can be aligned with the magnet or both against.These states will increase or decrease the applied field just as before.But if one proton HX is aligned with,and one against the applied field,there is no net change to the field experienced by HA and there are two possibilities for this.We therefore see a signal of double intensity for HA at the correct chemical shift,one signal at higher field and one at lower field.In other words,a 1:2:1 triplet.If there are more protons involved,we continue to get more complex systems,but the intensities can all be deduced simply from Pascals triangle,which gives the coefficients in a binomial expansion.Coupling is a through bond effect4JHH couplingW-coupling0 4JHH 3 HzGeminal 2JHHsaturated1016 Hzunsaturated03 HzVicinal 3JHHsaturated68 Hzunsaturated trans1416 Hzunsaturated cis811 Hzunsaturated aromatic69 HzLong-range 4JHHmeta13 Hzallylic12 HzSummaryFurther topics1.There are 13C-13C and 1H-13C couplings.1.1 13C-13C coupling does not cause splitting of signals into multiple peaks.Natural abundance of 13C 1.1%.1.2 In proton spectra we do not see coupling to 13C because of the low abundance of 13C.1.3 Hydrogen atoms attached to carbon can split 13C NMR into multiple peaks.However,it is possible to eliminate signal splitting by 1H-13C coupling by choosing instrumental parameters for the NMR spectrometer that decouple the proton-carbon interactions broadband proton decoupled.2.DEPT(distortionless enhanced by polarization transfer)13C Spectra3.Two-dimensional(2D)NMR techniques3.1 Proton-proton correlation COSY.3.2 Proton-carbon correlation HETCOR(heteronuclear chemical shift correlation spectroscopy).4.NOE(nuclear Overhauser effect)enables us to define the three dimensional stereochemistry of our molecules of interest.5.Solid-state NMR.5.NMR can not tell about absolute stereochemistry.紫外光谱的基本原理紫外光谱的基本原理常用的常用的紫外紫外-可见分光光度计可见分光光度计的波长范围是的波长范围是200800 nm紫外光谱紫外光谱是由分子中的是由分子中的价电子价电子经经近紫外光和可见光近紫外光和可见光照射发生照射发生电子能级跃迁电子能级跃迁而引而引起的起的吸收光谱吸收光谱，又称，又称电子光谱电子光谱紫外光谱紫外光谱是一种是一种分子吸收光谱分子吸收光谱紫外光谱紫外光谱常用于常用于共轭体系共轭体系的检测的检测远紫外远紫外(100200nm)可被氮可被氮,氧氧,二氧化碳二氧化碳,水吸收水吸收(真空紫外真空紫外)紫外光谱的产生紫外光谱的产生远紫外区远紫外区100200nm近紫外区近紫外区200400nm可见光区可见光区400800nm紫外紫外-可见光谱可见光谱UV-vis Spectroscopy电子跃迁的类型电子跃迁的类型E*n跃迁时所吸收的能量跃迁时所吸收的能量n*n*5000为为强吸收强吸收 =20005000为为中吸收中吸收 2000为为弱吸收弱吸收 丙酮丙酮对甲基苯乙酮对甲基苯乙酮各类化合物的电子跃迁各类化合物的电子跃迁饱和有机化合物饱和有机化合物饱和烃饱和烃(烷烃烷烃,环烷烃环烷烃)*吸收波长均在吸收波长均在远紫外区远紫外区含有含有孤对电子孤对电子(n电子电子)的饱和化合物的饱和化合物 *n *max=150 250 nm =100 3000饱和醇饱和醇 饱和醚饱和醚 饱和硫醚饱和硫醚 饱和胺饱和胺 卤代烷卤代烷CH4 max=125 nm CH3CH3 max=135 nm不饱和脂肪族化合物不饱和脂肪族化合物孤立的烯孤立的烯/炔炔ss*h *ss*max 5000），则可推测其结构必含有共轭体系，可把异，则可推测其结构必含有共轭体系，可把异构体范围缩小到共轭醛或共轭酮：构体范围缩小到共轭醛或共轭酮：小小 结结若在若在200750nm波长范围内无吸收峰，则可能是波长范围内无吸收峰，则可能是直链烷烃、环直链烷烃、环烷烃、饱和脂肪族化合物烷烃、饱和脂肪族化合物或或仅含一个双键的烯烃仅含一个双键的烯烃等。等。若在若在270350nm波长范围内有波长范围内有低强度吸收峰低强度吸收峰(10100),（n*跃迁跃迁），则可能含有一个），则可能含有一个简单非共轭且含有简单非共轭且含有n电子电子的生色团的生色团，如，如羰基羰基。若在若在20300nm波长范围内有波长范围内有中等强度的吸收峰中等强度的吸收峰则可能含则可能含苯苯环环，假设有精细结构的话，可能是苯环的特征吸收。，假设有精细结构的话，可能是苯环的特征吸收。若在若在210250nm波长范围内有波长范围内有强吸收峰强吸收峰，则可能含有则可能含有2个共轭个共轭双键双键；若在；若在260350nm波长范围内有波长范围内有强吸收峰强吸收峰，则说明该有机，则说明该有机物含有物含有3个或个或3个以上共轭双键个以上共轭双键。若该有机物的吸收峰延伸至若该有机物的吸收峰延伸至可见光区可见光区，则该有机物可能是，则该有机物可能是长链长链共轭共轭或或稠环化合物稠环化合物。Mass spectrometry weighs the moleculeA mass spectrometer has three basic components:1)something to volatilize and ionize the molecule into a beam of charged particles2)something to focus the beam so that particles of the same mass:charge ratio are separated from all others3)something to detect the particlesIn electron impact(E.I.)mass spectrometry the molecule is bombarded with highly energetic electrons that knock a weakly bound electron out of the molecule and leaves behind a radical cation.unstabletakes about 2 x 10-5 s to get to the detectorif the electron beam supplies more than exactly the right amount of energy to knock out the electron,the excess energy is dissipated by fragmentation of the radical cation to give a radical X and a cation Y+.Methods used to convert neutral molecules into cations:!electron impact(EI)!chemical ionization(CI)base peakM+The problem with E.I.is that for many radical cations even 2 x 10-5 s is too long,and all the molecular ions have decomposed by the time they reach the detector.In chemical ionization(C.I.)mass spectrometry the electron beam is used to ionize a simple molecule such as methane which in turn ionizes our molecule by collision and transfer of a proton.Mass spectrometry separates isotopesElement Carbon Chlorine Bromineisotopes 12C,13C 35Cl,37Cl 79Br,81Brrough ratio 1.1%13C(99:1)3:1 1:1The C.I.Mass spectrum give M+H+peak.Atomic comp