Competition between alkyl radical addition to carbonyl bonds and H-atom abstraction reactions
Competition between alkyl radical addition to carbonyl bonds and H-atom abstraction reactions
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DOI：
10.1039/b201883g
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发表时间：
2002-01-01
影响因子：
3.3
通讯作者：
Viskolcz, B
中科院分区：
文献类型：
--
作者：
Hippler, H;Viskolcz, B
甲基自由基加成到CH 2 O、CH 3CHO和（CH 3）（2）CO的活化能很小，分别约为28、29和40 kJ mol（-1）。对于将伯烷基自由基加成到甲醛和更高级的醛同系物中，我们发现活化势垒分别显著降低到（20 +/- 4）kJ mol（-1）和（27 +/- 2）kJ mol（-1）。一个几乎可以忽略不计的活化势垒约5 kJ mol（-1）被发现为二级自由基加成甲醛。在向甲醛中加入叔自由基的情况下，没有识别出活化屏障。通常，甲基加成到羰基的反应将与夺氢反应竞争。对于较大的烷基自由基除了羰基占主导地位的直接H-原子的抽象。本文给出了饱和烷氧基（CnH 2n +1 O.，其中n ≤ 5）自由基。计算的生成热与文献值估计的O-H键离解能吻合得很好。计算的Delta（f）Hdegrees的预期最大误差小于4 kJ mol（-1）。伯、仲和叔烷氧基值（GV）由从头计算Delta（f）H度-s计算，标准偏差为1.2 kJ mol（-1）。
The activation energies for the methyl radical addition to CH2O, CH3CHO, and (CH3)(2)CO, are small and around 28, 29, and 40 kJ mol(-1), respectively. For the addition of primary alkyl radicals to formaldehyde and higher aldehyde homologues, we find significantly lower activation barriers of (20 +/- 4) kJ mol(-1) and (27 +/- 2) kJ mol(-1), respectively. An almost negligible activation barrier of about 5 kJ mol(-1) was found for the secondary radical addition to formaldehyde. In the case of addition of a tertiary radical to formaldehyde no activation barrier could be identified. Generally, methyl addition reactions to carbonyls will compete with hydrogen abstraction reactions. For larger alkyl radicals addition to carbonyls dominates over the direct H-atom abstraction. A self-consistent set of standard enthalpies of formation Delta(f)Hdegrees for saturated alkoxy (CnH2n+1O., where n less than or equal to 5) radicals was obtained by the modified Gaussian-3(G3(MP2)//B3LYP) ab initio molecular orbital theory. The calculated heats of formations are in good agreement with literature values estimated from O-H bond dissociation energies. The expected maximum error of the calculated Delta(f)Hdegrees is less than 4 kJ mol(-1). The primary, secondary, and tertiary alkoxy group values (GV) were evaluated from the ab initio Delta(f)Hdegrees-s with a standard deviation of 1.2 kJ mol(-1).