Catechism #1398-1400, especially #1400.
1398 The Eucharist and the unity of Christians. Before the greatness of this mystery St. Augustine exclaims, "O sacrament of devotion! O sign of unity! O bond of charity!"237 The more painful the experience of the divisions in the Church which break the common participation in the table of the Lord, the more urgent are our prayers to the Lord that the time of complete unity among all who believe in him may return.
1399 The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. “These Churches, although separated from us, yet possess true sacraments, above all - by apostolic succession - the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy.” A certain communion in sacris, and so in the Eucharist, "given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged."238
1400 Ecclesial communities derived from the Reformation and separated from the Catholic Church, "have not preserved the proper reality of the Eucharistic mystery in its fullness, especially because of the absence of the sacrament of Holy Orders."239
It is for this reason that, for the Catholic Church, Eucharistic intercommunion with these communities is not possible. However these ecclesial communities, "when they commemorate the Lord’s death and resurrection in the Holy Supper . . . profess that it signifies life in communion with Christ and await his coming in glory."240
scborromeo.org/ccc/p2s2c1a3.htm