We introduce ChemComp, the first chemistry-focused benchmark for evaluating compositional multi-hop reasoning in large language models (LLMs). Our automated pipeline constructs benchmarks from proprietary or public data by integrating generative reasoning models, chemical named-entity recognition, and external knowledge bases to build knowledge graphs. Applied to recent chemistry literature, this approach minimizes overlap with LLM pretraining data. The resulting dataset comprises 1,188 multi-hop questions, refined through domain-expert feedback and robust evaluation protocols.Using ChemComp, we systematically compare LLM performance with and without retrieval augmentation, including an idealized gold-context scenario. Our results show that even state-of-the-art models struggle with compositional reasoning: retrieval significantly improves accuracy, yet reasoning errors persist even under perfect retrieval. These findings highlight the limitations of current LLMs and the critical role of retrieval-augmented methods in scientific reasoning. Furthermore, our pipeline is generalizable with fine-tuning, enabling the creation of challenging multi-hop reasoning benchmarks across domains and proprietary datasets.